Aqueous electrolyte lithium sulfur batteries

ABSTRACT

Provided are lithium sulfur battery cells that use water as an electrolyte solvent. In various embodiments the water solvent enhances one or more of the following cell attributes: energy density, power density and cycle life. Significant cost reduction can also be realized by using an aqueous electrolyte in combination with a sulfur cathode. For instance, in applications where cost per Watt-Hour (Wh) is paramount, such as grid storage and traction applications, the use of an aqueous electrolyte in combination with inexpensive sulfur as the cathode active material can be a key enabler for the utility and automotive industries, providing a cost effective and compact solution for load leveling, electric vehicles and renewable energy storage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. Nos. 13/440,847, filed Apr. 5, 2012, titled AQUEOUS ELECTROLYTELITHIUM SULFUR BATTERIES; which claims priority to U.S. ProvisionalPatent Application Nos. 61/585,589, filed Jan. 11, 2012, titled AQUEOUSLITHIUM-SULFUR BATTERY CELL, and 61/560,134, filed Nov. 15, 2011, titledAQUEOUS LITHIUM-SULFUR BATTERY. This application also claims priorityfrom U.S. Provisional Patent Application Nos. 61/623,031, filed Apr. 11,2012, titled AQUEOUS ELECTROLYTE LITHIUM SULFUR BATTERIES. Each of theseapplications is incorporated herein by reference in its entirety and forall purposes.

FIELD OF THE INVENTION

The present invention relates generally to the field of electrochemicalenergy storage and power delivery. In particular, the present inventionis directed to aqueous lithium-sulfur battery cells, including flowcells, and methods of making such cells.

BACKGROUND OF THE INVENTION

The lithium sulfur battery has a theoretical capacity of 1675 mAhg⁻¹ andapproximately 2300 Wh/kg. The low cost and exceptionally high specificcapacity of sulfur renders it an especially attractive battery cathodematerial for large-scale energy storage, including electric vehicle andgrid storage applications. Yet after more than twenty years of researchand development at various battery companies and scientific institutionsworldwide, key technical problems with the sulfur electrode haveprecluded meaningful commercialization of the Li-S battery.

SUMMARY OF THE INVENTION

In one aspect the invention provides an aqueous lithium sulfur batterycell having an anode structure comprising an electroactive material, acathode comprising a solid electron transfer medium, an aqueouselectrolyte in contact with the electron transfer medium, and activesulfur species in contact with the aqueous electrolyte, and wherein theanode electroactive material is isolated from direct contact with theaqueous electrolyte. Notably, while the anode electroactive material isisolated from touching (i.e., directly contacting) the aqueouselectrolyte, it is nonetheless configured in the anode structure to bein lithium ion communication with the aqueous electrolyte. Moreover,because the aqueous electrolyte does not touch the anode electroactivematerial but does directly contact the cathode the term “aqueouscatholyte” (or more simply “catholyte”) is used interchangeably with theterm “aqueous electrolyte”.

In various embodiments the aqueous electrolyte is electroactive in thatit contains dissolved active sulfur species that undergo electrochemicalredox at the cathode during discharge and charge. Without limitation,the dissolved redox active sulfur species may include sulfide anions(S²⁻), hydrosulfide anions (HS⁻), and polysulfide anions including S_(x)²⁻ with x>1 and hydropolysulfide anions (HS_(x) ⁻ with x>1), andcombinations thereof.

In accordance with the present invention, the amount of water in thecatholyte is significant (i.e., not merely a trace amount). In variousembodiments the volume percent of water relative to the total liquidsolvent volume in the catholyte is greater than 5%, 10%, 20%, 30%, 40%,50%, 60%, 70%, 80%, and greater than 90%. In certain embodiments wateris the only liquid solvent in the catholyte (i.e., water constitutes100% of the solvent volume of the catholyte). In various embodimentswater is the main liquid solvent in the catholyte. By use of the termmain liquid solvent, it is meant that the volume percent of water in thecatholyte is greater than the volume percent of any other liquidsolvent.

Water has unique properties. In the aqueous sulfur catholyte solutionsdescribed herein, the presence of water provides a number of benefits,including high solubility for active sulfur species, including lithiumsulfide (Li₂S), very high ionic conductivity even at high sulfurconcentrations, and fast dissolution kinetics. The combination of highsolubility, high conductivity, and fast dissolution kinetics providescompelling lithium sulfur battery performance.

Accordingly, in various embodiments the cell is fabricated with anaqueous catholyte having a high concentration of active sulfur speciesalready dissolved therein. In other words, the cell has a significantamount of dissolved active sulfur species adjacent the electron transfermedium even before the cell has been initially operated (e.g., initiallydischarged) and by this expedient the fast electro-kinetics of solutionphase redox can be used to advantage, especially, but not exclusively,for applications that require high current drain immediately upon startup. For instance, in various embodiments, prior to initially operatingthe cell, the active sulfur concentration in the aqueous electrolyte isgreater than 0.5 molar sulfur, 1 molar sulfur, 2 molar sulfur, 3 molarsulfur, 4 molar sulfur, 5 molar sulfur, 6 molar sulfur, 7 molar sulfur,8 molar sulfur, 9 molar sulfur, 10 molar sulfur, 11 molar sulfur, orgreater than 12 molar sulfur. Herein and in the claims, by the use ofthe term “molar sulfur” it is meant the number of moles of sulfur perliter of electrolyte. Moreover, by use of the phrase “just prior toinitially operating the cell” or “prior to initial cell operation” it ismeant, herein and in the claims, to mean the first (i.e., initial)electrochemical operation activated by the user and specifically itrefers to one or the other of cell discharge or cell charge, whicheveris caused to occur, by the user, first. In other words, incidentalself-discharge (e.g., on storage) does not qualify herein or in theclaims as an initial cell operation.

Moreover, because it can be difficult to identify the precise chemicalnature of the various active sulfur species existing in the catholytesolution, the composition of the active species in the catholyte (i.e.,active catholyte composition) is sometimes expressed herein, and in theclaims, in terms of an “active lithium sulfur stoichiometric ratio” ormore simply an “active stoichiometric ratio” which is the ratio ofactive sulfur to active lithium dissolved in the electrolyte, andrepresented by the general formula Li₂S_(x). Furthermore, it should beunderstood that the “active stoichiometric ratio” as used herein isexclusive of any non-active lithium salts and/or non-active sulfur saltsthat may be added to the electrolyte for any purpose, including, e.g.,to enhance lithium ion conductivity in the case of, e.g., a non-activeLiCl salt, or a non-active sulfur containing salt such as, e.g.,LiSO₃CF₃.

Accordingly, in various embodiments, the active lithium sulfurstoichiometric ratio in the catholyte prior to, in particular just priorto, initial cell operation is Li₂S; Li₂S_(x) (x>1); Li₂S_(x) (1<x<5);Li₂S₅; or Li₂S_(x) (x>5), and the concentration of the dissolved activesulfur species is typically significant, e.g., greater than 1 molarsulfur. For instance, in particular embodiments, especially for cellsusing a lithium metal or lithium alloy as the electroactive anodematerial, the active stoichiometric ratio just prior to initial celloperation is Li₂S_(x) with the following range for x: 2≦x≦5, and theactive sulfur concentration is between 10 to 17 molar sulfur. Forexample, a catholyte composition having an active stoichiometric ratioof about Li₂S₄, and at concentrations greater than 10 molar sulfur(e.g., 11, 12, 13, 14, 15, 16 or 17 molar sulfur). In another particularembodiment, especially useful for cells which are fabricated in thefully or mostly discharged state (e.g., having an anode electroactivematerial that is devoid of active lithium), the active stoichiometricratio of the catholyte just prior to initial cell operation is Li₂S, andthe active sulfur concentration is typically greater than 1 molarsulfur, and preferably greater than 2 molar sulfur, and more preferablygreater than 3 molar sulfur (e.g., 3 molar, 4 molar, or 5 molar sulfur).

Another advantage of the aqueous catholyte is that it may serve as amedium into which high concentrations of fully or partially reducedactive sulfur species (e.g., Li₂S) may be quickly dissolved duringcharge. By this expedient high capacity cells in accordance withembodiments of the instant invention may be deeply discharged repeatedlysince the cell reaction product on discharge (e.g., Li₂S) is readilydissolved and therefore more readily oxidized on charge. Thus, invarious embodiments, the cell is formulated and operated such that asignificant portion of the sulfur ampere-hour capacity, at the end ofdischarge, is present in the form of solid phase lithium sulfide.

Furthermore, the combination of high solubility and fast dissolutionkinetics of Li₂S in water also enables a practical method of making anaqueous lithium sulfur cell that is assembled in the fully dischargedstate, and which makes use of alternative anode electroactive materialsthat are different than that of lithium metal, such as carbonintercalation materials, alloys (e.g., of silicon) and combinationsthereof such as carbon silicon composites. For example, one method inaccordance with the present invention involves: i) providing a carbonanode in the fully discharged state (i.e., entirely un-intercalated);ii) providing an aqueous sulfur catholyte comprising water and dissolvedlithium sulfide; iii) providing a cathode comprising an electrontransfer medium for electrochemical oxidation of dissolved lithiumsulfide; iv) configuring the anode, catholyte and cathode into a batterycell; and iv) charging the battery cell. Accordingly, in variousembodiments the instant cell comprises both dissolved lithium sulfideand a significant amount of solid phase lithium sulfide in contact withthe aqueous electrolyte. For instance, in various embodiments the molarquantity of active sulfur as solid phase lithium sulfide is greater thanthat of active sulfur dissolved in the electrolyte by a factor of atleast 2, or at least 3, or at least 5 or at least 10. Moreover, in thesame or separate embodiments, the full charge capacity of the cell justprior to initial cell operation is derived from the ampere-hour capacityof dissolved active sulfur species in the catholyte combined with theampere-hour capacity of solid phase lithium sulfide. Furthermore, in thesame or separate embodiments upon cell fabrication and just prior toinitial cell operation the anode electroactive material is substantiallydevoid of active lithium, and the initial cell operation is to chargethe battery. For example, the anode electroactive material may be anintercalation material capable of electrochemically intercalatinglithium upon electro-reduction in the presence of lithium ions, or analloying material capable of electrochemically alloying with lithiumupon electro-reduction in the presence of lithium ions, or a materialcapable of forming a lithium inter-metallic phase upon electro-reductionin the presence of lithium ions. For example, in particular embodimentsthe anode electroactive material is an intercalating carbon, silicon, ora composite of said silicon and carbon.

In applications where high pulse power and size are paramountperformance benefit may be gained by taking advantage of the facileelectro-kinetics of solution phase redox in combination with the highsolubility of polysulfide species in water. For instance, in variousembodiments, the cell is formulated and operated such that theampere-hour capacity in the cell, at full state of charge, is solelypresent as dissolved active sulfur species in the catholyte. Inparticular the cell may be fabricated in the fully charged state devoidof solid phase active sulfur (e.g., devoid of elemental sulfur).

The use of water as a catholyte solvent clearly provides considerablebenefit, but it also presents significant challenges in a lithium-sulfurbattery. In particular, the use of water is constrained by itsreactivity with electroactive lithium materials (e.g., lithium metal).Accordingly, the present invention makes use of lithium anode structureswherein the electroactive lithium is isolated from contacting theaqueous sulfur catholyte. In various embodiments, a protected lithiumelectrode is employed which contains a lithium electroactive materialprotected from the external environment by a substantially imperviouslithium ion conductive protective membrane architecture. Thus inaccordance with the instant invention the aqueous catholyte is disposedin the cell such that it directly contacts the electron transfer mediumbut does not contact the electroactive material of the anode (e.g.,lithium metal or carbon intercalation material).

A further challenge to the use of water in a lithium-sulfur cell is thehydrolysis of dissolved lithium sulfide (Li₂S) in the catholyte and theresulting generation of hydrogen sulfide (H₂S). According to someembodiments of the present invention, a lithium-sulfur cell can comprisea housing configured to contain and withstand the pressure of such gasgeneration to maintain cell integrity and safety. According to furtherembodiments, the pH of the electrolyte (catholyte) can be adjusted toreduce or prevent Li₂S hydrolysis. This is particularly achieved withbasic pHs, for example greater than 7, or from about 9 to 12 and up to14. However, the invention is not limited to basic electrolytes, and itis contemplated herein that the pH may be adjusted to values below pH 7(i.e., acidic) or about pH 7 (i.e., neutral catholyte) using acidicsalts and buffering agents.

Further relating to suitable electrolyte/catholyte formulations inaccordance with the present invention, compositions and methods areprovided to enhance contact between the aqueous electrolyte and thecathode electron transfer medium, for example an electronicallyconductive matrix such as a carbon or metal mesh, foam or other highsurface area, typically porous, structure. Such improved contactenhances utlilization and rate performance of the cell.Electrolyte/catholyte compositions in this regard can include asurfactant to wet the catholyte to the conductive matrix. Also oralternatively, the matrix can be surface treated prior to contact withthe electrolyte to enhance wetting, for example being soaked in awetting agent, followed by displacement of the wetting agent with theaqueous catholyte solution of polysulfides. Still further in thisregard, the catholyte may include dissolved organosulfur as a cathodeactive material. The organosulfur compound or compounds can self-wet tothe cathode electron transfer matrix

Another aspect of the present invention relates to the challengepresented in an aqueous lithium-sulfur battery with regard to thevoltage stability window of water and the active sulfur (e.g., dissolvedpolysulfide) redox potentials. In order to expand the redox potentialwindow in which an aqueous lithium-sulfur battery cell may operatewithout generating hydrogen and oxygen from the water in theelectrolyte, battery cells in accordance with embodiments of the presentinvention may include a material with a high overpotential for hydrogen(H₂) and/or oxygen (O₂) in the cathode, in particular as or as part ofthe electron transfer medium of the cathode. For example, a cathodematrix can be formed from a metal with a high overpotential for H₂, suchas lead (Pb). Or, a metal with a high overpotential for H₂ (and/or O₂)can be coated as an exterior layer on an underlying matrix structure(also sometimes referred to herein as a “core” or “core structure”). Insome such embodiments, the underlying matrix structure can be anelectronic insulator (e.g., a glass or polymer) so that discontinuitiesin the coating do not result in the generation of hydrogen (or oxygen)gas at an underlying conductor's surface. By providing a cathodeelectron transfer medium with a high overpotential for H₂ and/or O₂battery cells in accordance with the present invention have an extendedoperating potential range, beyond that of the potential window of water.

Yet another aspect of the present invention relates to compositionsdefining the exterior surface of the cathode electron transfer medium(e.g., matrix) that electro-catalyze sulfur redox but also have a highoverpotential for H₂, such as metal sulfides (e.g., lead sulfide,cadmium sulfide, cobalt sulfide and nickel sulfide) and in this way canprovide both catalysis and high overpotential for H₂ as described above.Such coatings should allow effective electron tunneling so as not todisrupt the electron transfer function of the matrix. The coatings maybe applied to a conventional conductive matrix material, such as carbon,or to a matrix material having a high overpotential for H₂, such asdescribed above.

In yet another aspect the present invention relates to catholyteformulations including the incorporation of one or more non-aqueoussolvents for particular benefit. Non-aqueous solvents suitable for useherein to improve performance of the instant aqueous lithium sulfurbattery cells include aprotic and protic organic solvents and ionicliquids.

In particular embodiments the aqueous catholyte comprises water and aprotic solvent that is non-aqueous, especially protic organic solventsthat are capable of dissolving a significant amount of Li₂S (e.g.,methanol). Addition of the non-aqueous protic solvent is particularlyuseful in cells that may be operated at temperatures below the freezingtemperature of water and yet still require high solubility for lithiumsulfide. Accordingly, in various embodiments the catholyte is formulatedwith an amount of a non-aqueous protic solvent (e.g., ethylene glycol)sufficient to achieve a freezing point temperature (i.e., melttemperature) below a desired value; for example, below −5° C., −10° C.,−20° C., −30° C. or −40° C.

While the invention has generally been described with reference to anelectroactive catholyte (i.e., a catholyte containing dissolved activesulfur species) and/or electroactive fully reduced solid phase lithiumsulfide loaded in the cathode, the invention is not limited as such, andit is contemplated that fully oxidized solid phase electroactive sulfur(e.g., elemental sulfur) or active organosulfur compounds may beincorporated in the cell during fabrication as an exclusive source ofactive sulfur or in combination with an electroactive sulfur catholyte.Notwithstanding the aforementioned sulfur containing cathodeconfigurations, in various embodiments the cell is fabricated absentelemental sulfur, and the cathode is, thereby, devoid of elementalsulfur just prior to initial cell operation.

The invention also relates to methods of manufacture of aqueouslithium-sulfur battery cells. In one aspect, such a method involvesde-oxygenating the catholyte and forming and sealing the cell in aninert or reducing environment devoid of molecular oxygen (e.g., anitrogen environment) in order to reduce or eliminate free oxygen (O₂)in the catholyte solution. In this way the irreversible oxidation ofsulfur species in the aqueous catholyte (e.g., oxidation leading toinsoluble thiosulfates) and the resultant loss of active material, isreduced or avoided.

In various embodiments the instant cells are self-contained and sealedin a hermetic casing wherein the entirety of the cell capacity isderived from electroactive sulfur and electroactive lithium disposed inthe casing during cell manufacture. These fully sealed cells may be ofthe primary or secondary type.

In other embodiments the instant cells are configured in a battery flowcell system, wherein an aqueous sulfur catholyte is caused to flow,and/or circulate, into the cell, and, in various embodiments, through aninter-electrode region between the lithium anode and the cathodeelectron transfer medium. In some embodiments both the aqueous catholyteand the electroactive lithium are flowable and during operation arecaused to flow through the cell.

It should be understood that aqueous lithium-sulfur battery cells inaccordance with the present invention are not merely different fromconventional non-aqueous Li—S battery cells by their substitution of anon-aqueous electrolyte solvent with an aqueous electrolyte solventsystem. The use of water in the electrolyte results in a solvent systemthat is not just a spectator, but actually participates in theelectrochemical reactions at the cathode, reacting to form and dissolvenew species. The present invention is therefore directed to an entirelynew class of battery cells having entirely different chemistry thanconventional Li—S battery cells (as evidenced by the dramatic differencein their voltage profiles), and to the formulation, engineering,operation and manufacturing challenges associated therewith.

These and other aspects of the present invention are described in moredetail, including with reference to figures, in the description whichfollows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section of a battery cell in accordance withvarious embodiments of the present invention.

FIGS. 2A-B illustrates an electron transfer medium in accordance withvarious embodiments of the present invention.

FIG. 3 is a qualitative illustration of a Pourbaix diagram for water andactive sulfur species in catholyte in accordance with the presentinvention.

FIG. 4 is a photograph comparing the solubility of Li₂S in water withthat in a non-aqueous solvent.

FIGS. 5A-D illustrate various alternative configurations of a protectivemembrane architecture in accordance with the present invention.

FIG. 6 is a schematic cross section of a battery flow cell system inaccordance with an embodiment of the present invention.

FIG. 7 is a schematic cross section of a battery flow cell system inaccordance with an alternative embodiment of the present invention.

FIG. 8 is a plot comparing the cyclic voltammogram of an aqueous lithiumsulfur cell in accordance with an embodiment of the present inventionand a cell without active sulfur.

FIG. 9 is a cyclic voltammetric plot comparing the potential window foraqueous lithium sulfur cell operation using two different cathodematerials.

FIG. 10 is a cyclic voltammetric plot comparing alternative aqueouslithium sulfur cell embodiments in accordance with the presentinvention.

FIG. 11 is a voltage vs. time cycling profile and a capacity vs. cyclenumber profile for an aqueous lithium sulfur cell in accordance with thepresent invention.

FIG. 12 is a voltage vs. capacity profile for an aqueous lithium sulfurcell in accordance with the present invention.

FIG. 13 is a voltage vs. time cycling profile and a capacity vs. cyclenumber profile for an aqueous lithium sulfur cell in accordance with anembodiment of the present invention.

FIG. 14 is a voltage vs. time cycling profile and a capacity vs. cyclenumber profile for an aqueous lithium sulfur cell in accordance with anembodiment of the present invention.

FIG. 15 is a voltage vs. time cycling profile and a capacity vs. cyclenumber profile for an aqueous lithium sulfur cell in accordance with anembodiment of the present invention.

FIG. 16 is a voltage vs. time cycling profile for an aqueous lithiumsulfur cell in accordance with an embodiment of the present invention.

FIG. 17 is a voltage vs. time cycling profile and a capacity vs. cyclenumber profile for a lithium sulfur cell in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Reference will now be made in detail to specific embodiments of theinvention. Examples of the specific embodiments are illustrated in theaccompanying drawings. While the invention will be described inconjunction with these specific embodiments, it will be understood thatit is not intended to limit the invention to such specific embodiments.On the contrary, it is intended to cover alternatives, modifications,and equivalents as may be included within the spirit and scope of theinvention. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. The present invention may be practiced without some or all ofthese specific details. In other instances, well known processoperations have not been described in detail so as to not unnecessarilyobscure the present invention.

A lithium sulfur cell in accordance with various embodiments of theinstant invention is shown in FIG. 1. The cell 100 includes a cathode110 comprising an electron transfer medium, a protected lithium anode120 and an aqueous electrolyte in contact with the electron transfermedium and also in contact with an exterior surface of the protectedlithium anode.

The protected lithium anode 120 includes a lithium electroactivematerial layer 122 and a substantially impervious lithium ion conductingprotective membrane architecture 126 on the surface of the lithiumactive layer 122. The membrane architecture is substantially imperviousto water and has a first surface chemically compatible in contact withthe lithium electroactive layer and a second surface, opposing thecathode, which is chemically compatible in contact with water, and inparticular chemically compatible in contact with the catholyte employedin the cell. In some embodiments the cell further includes a porousseparator material layer 130 interposed between the cathode and theprotected anode, and containing in its pores at least a portion of theaqueous electrolyte (i.e., aqueous catholyte). In other embodiments thecell is absent a separator and it is contemplated herein that themembrane architecture second surface directly contacts the cathode,which, in said embodiments, is generally porous with catholyte fillingthe pore spaces.

The cathode 110 includes a solid electron transfer medium having an“exterior surface” that is chemically compatible in contact with thecatholyte and on which dissolved active sulfur species areelectro-reduced during cell discharge and electro-oxidized on charge.With reference to FIGS. 2A-B, in various embodiments the electrontransfer medium 200A/200B may be a porous three-dimensional structure200A or planar 200B and substantially dense or otherwise porous (e.g., aplanar mesh). Whether dense or porous, the medium should be sufficientlyelectronically conductive to support the electrical current through thecell and its exterior surface capable of supporting the electrontransfer current. When porous, the solid electron transfer medium maytake the form of a porous matrix such as a woven or non-woven fibernetwork (e.g., a metal or carbon fiber cloth or paper) or a throughporous monolithic solid body (e.g., a metal or carbon foam). Whenplanar, the medium may simply be a metal or carbonaceous sheet or foilor open mesh of sufficient thickness and conductivity to beself-supporting, or the planar medium may be a composite having a firstlayer, typically thin and electronically conductive, that defines theexterior surface and a second layer serving as a substrate support, andoptionally further providing current collection when electronicallyconductive.

The electron transfer medium has an exterior surface that may be porousor dense but is defined, at least in part, by a material that, incontact with the catholyte, facilitates electron transfer, and, inparticular, facilitates electrochemical redox of active sulfur species.Continuing with reference to FIGS. 2A-B, in various embodiments theelectron transfer medium 200A/200B is a porous matrix composed of a corecomponent (i.e., underlying matrix structure) 210A/210B having anexterior layer component 220A/220B that provides the exterior surface incontact with the catholyte. The core component generally providessubstrate support and may, when conductive, facilitate currentcollection, whereas a primary function of the exterior layer is toprovide some benefit to the electrochemical performance, and inparticular that pertaining to electron transfer. The exterior layer maybe porous or dense. In various embodiments, a dense exterior layer isalso preferably contiguous and therefore substantially covers the coresurface in its entirety. In other embodiments, a porous exterior layeris suitable, especially when the surface composition of the core iscompatible with the catholyte and does not catalyze hydrogen evolution,as described in more detail below. Furthermore, when porous, theexterior layer may include high surface area particles thatelectro-catalyze sulfur redox and/or increases the effective surfacearea for electrical benefit.

In some embodiments the core, electronically conductive, supportscurrent collection, while the exterior layer primarily serves to supportand preferably enhance electrochemical sulfur redox. In otherembodiments the core is electronically insulating and the exterior layerprovides electron transfer and may provide some or all of the currentcollector function. The insulating core may be composed of any suitableinsulating material of sufficient mechanical integrity and is preferablyalthough not necessarily chemically compatible in contact with thecatholyte. In certain embodiments the exterior layer is dense andsubstantially free of defects that otherwise would allow water from theelectrolyte to seep into contact with the core material, and potentiallyreduce its strength or mechanical integrity. To prevent this fromhappening, in preferred embodiments the core material is also chemicallycompatible in contact with the catholyte and even more preferably is amaterial that does not swell or lose mechanical strength when in contactwith water, and specifically does not mechanically degrade or changeshape if exposed to the active electrolyte. In various embodimentsadditional layers may be incorporated between the insulating orconductive core and the exterior layer to support current collectionand/or provide or improve interface compatibility and/or adhesion. Forexample, the insulating core of an underlying matrix structure may havea first metal coating (e.g., aluminum) serving as an intermediary layerto provide current collection and a second coating covering the aluminumthat defines, in whole or in part, the exterior surface for the purposeof facilitating sulfur redox.

The electron transfer medium may be uncatalyzed, relying solely on themedium material (e.g., carbon) to facilitate the electrochemical redoxreactions, or, in some embodiments, the electron transfer medium maycontain a catalyst on its surface, such as a particulate catalyst or thecatalyst may be formed on the underlying carbon or metal matrix as acoating. In some embodiments the exterior layer is a porous high surfacearea film composed of electronically conductive particles (e.g., highsurface area carbons including nano-carbons, carbon blacks andfunctionalized carbons) that preferably electro-catalyze at least one orboth of electro-reduction and electro-oxidation of active sulfur. Inother embodiments, as described in more detail below, the exterior layermay be a dense, preferably thin, electronically conductive layer, suchas a thin dense film of a metal, metal alloy, or metal compound (e.g., ametal sulfide) for the purposes of providing one or more of electronicconduction, facilitation of sulfur redox, and expansion of the voltagestability window of the catholyte, as described in more detail below.

With regard to the voltage window of the catholyte, a significant issuemay arise during discharge once the cell voltage drops below a “criticalvoltage” corresponding to the thermodynamic potential for waterreduction the cell electrochemistry is made complicated by thepotentiality of water decomposition, and in particular H₂ evolution. Theissue is illustrated pictorially with reference to FIG. 3, showing aPourbaix diagram of water compared to an illustrative Pourbaix diagramof sulfur redox without assigning voltages to the sulfurelectro-reduction/oxidation reactions. As can be seen in theillustration, the critical voltage varies with pH. For instance at pH 12the critical voltage versus lithium is about 2.3 Volts and decreaseswith increasing pH values, reaching about 2.2 Volts at pH 14. Asillustrated, albeit quite qualitatively, at cell voltages below thevoltage stability window of water (i.e., below the critical voltage)there exist significant active sulfur ampere-hour capacity; however, thepracticality of harnessing that capacity is complicated by waterdecomposition.

In this regard, the present invention provides cathode structures havingelectron transfer mediums that enable the instant cells to be dischargedto voltages beyond the thermodynamic potential for water reduction, andthereby efficiently harness the additional ampere-hour capacity whichexists at cell voltages below the critical voltage, and preferably do sowithout evolving any H₂. Accordingly, in various embodiments, theexterior surface of the electron transfer medium provides at least adual functionality: a first function to facilitate electrochemicalreduction/oxidation of the active sulfur species and a second functionto inhibit hydrogen evolution. For example, the exterior surface may bedefined in whole or in part by a material that facilitates sulfur redoxbut has a high overpotential for H₂ evolution. By this expedient thecell may be efficiently discharged to voltages below the criticalvoltage without evolving H₂. Preferably the exterior surface has anoverpotential of at least 50 mV beyond the thermodynamic potential ofwater reduction, and in embodiments disclosed herein the overpotentialis beyond 100 mV, beyond 200 mV, beyond 300 mV, beyond 400 mV, beyond500 mV, beyond 600 mV, and in certain embodiments beyond 700 mV andbeyond 800 mV. For instance, with regard to cell voltages, the use ofhigh overpotential electron transfer medium allows cells of the instantinvention to be discharged to cell voltages below 2.4 V, preferablybelow 2.3 V, even more preferably below 2.2V, below 2.1V and yet evenmore preferably below 2.0 V, below 1.9 V, below 1.8 V, below 1.7 V,below 1.6 V and below 1.5V.

Accordingly, in various embodiments at least a portion and in certainembodiments the entirety of the exterior surface of the electrontransfer medium is defined by a material having a high overpotential forH₂ evolution. Suitable classes of such materials include metals, metalalloys (e.g., amalgams), and metal chalcogenides, especially metalsulfides. Particularly suitable metals include lead, cadmium, indium,nickel, gallium, tellurium, manganese, and zinc, or some combinationthereof. Particularly suitable metal alloys include amalgams.Particularly suitable metal sulfides include cobalt sulfide, coppersulfide, nickel sulfide, and zinc sulfide, or some combination thereof.The thickness of the exterior layer is a tradeoff between burdening thecell with extra weight and other considerations such as one or more ofthe composition of the core material, mechanical strength, conductivityand coating process. For instance, in embodiments the exterior layerthickness may be in the range of 50 microns to values below 1 micron(e.g., about 0.5 microns or 0.25 microns). With regard to a metalsulfide exterior layer the composition may vary gradually or discretelyacross its thickness. For example, as described the exterior layer maybe formed in two steps, first the metal of the metal sulfide may becoated, directly or indirectly, onto the core component surface, andthen the metal layer sulfidized to form a thin layer of metal sulfide,which in embodiments may be thin and dense, for example less than 10 nm,e.g., about 5 nm, about 2 nm or about 1 nm. Such thin films are alsoself-healing in that if a portion of the metal sulfide film were toflake off or start cracking, the underlying metal layer surface wouldsubsequently react with sulfur in the catholyte to reform the sulfidefilm.

In a particular embodiment the porous electron transfer medium iscomposed of a core component (e.g., a glass fiber mat) and a metalsulfide exterior layer (e.g., cobalt sulfide or lead sulfide). The corecomponent may be electronically insulating, and the metal sulfide formedby first applying a layer of the metal of the sulfide on the core (e.g.,coating the core with lead) and then sulfidizing the metal coated coresurface via treatment in a sulfur containing environment. The metallayer may be applied using coating methods applicable for bothelectronically conductive and insulating core structures, as are knownin the art generally, including evaporation, dip coating from the melt,electro-deposition and electroless deposition. Alternatively, the corecomponent may itself be composed of a material with a high overpotentialfor H₂ (e.g., a porous lead or porous cobalt matrix). However, the useof a heavy metal core material may unduly burden the overall cellweight, so in preferred embodiments the core material is composed of amaterial of light weight and preferably low density, such as carbon(e.g., graphitic like fibers or carbon foams), light weight metals suchas aluminum, or inorganic materials such as silica or other glasses, ororganic materials such as polymers (e.g., polymer fibers). Hollow coresare also contemplated herein for providing an exceptional lightweightadvantage. Carbon is a particularly useful core material as it can befabricated into a number of porous formats including porous fibermatrices and foams, and is also electronically conductive and thuscapable of supporting current collection, which enables the use ofexceptionally thin exterior layers. For example, less than 5 micronthick, preferably less than 1 micron, and even more preferably less than0.5 micron, and yet even more preferably the thickness of the exteriorlayer is less than 0.25 microns. In the same or separate embodiments,especially when the core is electronically insulating, an intermediateelectronically conductive layer (e.g., an aluminum layer) may be appliedas a coating between the core and the exterior layer to provide currentcollection support or the exterior layer itself may be of sufficientthickness to support the electrical current. For instance anintermediate metal layer such as aluminum having thickness between 0.25microns and 10 microns, and more preferably between 0.5 microns and 5microns; for example, about 0.5 microns, about 1 micron, about 2microns, about 3 microns, about 4 microns, and about 5 microns.Thereafter the exterior layer applied to the surface of the intermediarylayer using one or more of the aforementioned coating techniques, orother coating techniques generally known in the arts.

In various embodiments, the composition of the exterior surface may bemodified via surface treatments, and in particular, sulfidization toform a sulfide composition suitable for supporting, and preferably,electro-catalyzing sulfur redox. The step of sulfidization may becarried out in-situ within the cell by using a sulfur based catholyte.And while in-situ processing has the clear advantage of simplicity, italso leads to a concomitant loss in active sulfur cell capacity, sinceat least some of the sulfur that would have otherwise provided cellcapacity is consumed by the sulfidization treatment, and for highsurface area porous matrix structures, the loss of active sulfurcapacity can be significant. Accordingly, in preferred embodiments forsulfidizing porous matrix structures, the sulfidization step is carriedout ex-situ in a sulfur environment remote from the cell. For instance,the core material composed of the metal of the metal sulfide, or a corecomponent coated with said metal may be placed in a bath of an aqueouslithium polysulfide solution similar to or identical in nature to thecatholyte utilized in the cell, and allowed to stand in the bath for atime sufficient to form a substantially dense and pore free metalsulfide film.

Continuing with reference to FIG. 1 the cathode 110 may be assembled inthe cell devoid of elemental solid sulfur, and the entirety of thesulfur capacity loaded into the cell via the catholyte or solid phaseLi₂S. Alternatively, the cathode may include some form of solidelemental sulfur, including crystalline sulfur, amorphous sulfur,precipitated sulfur, and sulfur solidified from the melt. Elementalsulfur includes the various polyatomic molecules of sulfur, especiallythe octasulfur allotrope characterized as cyclo-S₈ ring, and polymorphsthereof such as α-octasulfur, β-octasulfur, and γ-octasulfur. Forexample, elemental sulfur (in the form of sulfur particulates includingnano-sized sulfur particles) may be incorporated in the cell as amaterial component of the cathode, wherein, e.g., the sulfur may beadmixed with high surface area or activated carbon particles and anappropriate binder (PTFE, PvDF and PEO) for adhering the materialcomponents in a suitable liquid carrier for formulating a slurry to becoated onto or impregnated into the porous matrix structure. Slurryformulations, with or without solid elemental sulfur, and coatingmethods suitable for use herein for incorporating solid phase activesulfur into the cathode are described in U.S. Pat. Nos.: 6,030,720,6,200,704, and 6,991,662, each of which is hereby fully incorporated byreference for all that they describe, and in particular for the slurryformulations and coating methods described. In the same or separateembodiments the active sulfur in the cathode may be or further includeelectroactive organosulfur compounds, including those described in U.S.Pat. Nos.: 4,833,048; 4,917,974; 5,162,175; 5,516,598, hereby fullyincorporated by reference, in particular for their disclosure relatingto organosulfur compound composition and use.

In alternative embodiments, the cells may be assembled having all of thesulfur capacity loaded in the cathode, e.g., in the form of elementalsulfur. In other embodiments, sulfur is present in the cathode as asolid phase electroactive material as well as in the aqueous catholytein the form of dissolved polysulfide species. In some embodiments thecell is assembled using a cathode that is loaded with solid phase Li₂S,and by this expedient, the cell may be assembled in the fully orpartially discharged state, wherein all or a portion of the activelithium is stored in or nearby the cathode during cell assembly. The asassembled cell is then subsequently charged, e.g., to full chargecapacity, prior to initial discharge.

Aqueous Sulfur Catholyte

In accordance with the instant invention, the aqueous catholyte containsa significant amount of water (i.e., not merely a trace amount), and thecatholyte is disposed in the cell such that it directly contacts thecathode. In certain embodiments water serves as the main liquid solventof the sulfur catholyte (i.e., electrolyte in contact with the sulfurcathode), and in particular embodiments water is the only catholytesolvent.

In accordance with the instant invention a significant (non-trace)amount of water is incorporated in the catholyte. In various embodimentsthe volume percent of water in the catholyte relative to the totalliquid solvent volume is greater than 5%, 10%, 20%, 30%, 40%, 50%, 60%,70%, 80%, and greater than 90%. In certain embodiments water is the onlyliquid solvent in the catholyte, and in particular embodiments thereofwater is the only liquid solvent (i.e., water constitutes 100% of thesolvent volume of the catholyte). In various embodiments water is themain solvent in the catholyte.

Water has unique properties. In aqueous sulfur catholyte solutions,water chemically interacts with the active sulfur species to provide anumber of benefits. In various embodiments the water serves as a mediuminto which a large concentration of active sulfur species may bedissolved (e.g., including sulfide anion (S²⁻), polysulfide anion (S_(x)²⁻ with x>1), hydrosulfide anion (HS⁻), polyhydrosulfide anion (HS_(x) ⁻with x>1) and combinations thereof). In various embodiments, thecatholyte composition just prior to initially operating the cell, whichis typically the catholyte composition upon cell fabrication andsealing, includes a significant concentration of dissolved active sulfurspecies. For instance, an active sulfur concentration in the catholyteof greater than 0.5 molar sulfur, greater than 1 molar sulfur, greaterthan 2 molar sulfur, greater than 3 molar sulfur, greater than 4 molarsulfur, greater than 5 molar sulfur, greater than 6 molar sulfur,greater than 7 molar sulfur, greater than 8 molar sulfur, greater than 9molar sulfur, greater than 10 molar sulfur, greater than 11 molarsulfur, greater than 12 molar sulfur, greater than 13 molar sulfur,greater than 14 molar sulfur, greater than 15 molar sulfur, greater than16 molar sulfur or greater than 17 molar sulfur may be used.

Moreover, because it can be difficult to identify the precise chemicalnature of the various active sulfur species existing in the catholytesolution at any given time during the course of discharge or charge, thecomposition of the active species in the catholyte is sometimesexpressed herein, and in the claims, in terms of an “activestoichiometric ratio” which is the ratio of active sulfur to activelithium dissolved in the electrolyte, and that ratio is represented bythe general formula Li₂S_(x). Furthermore, it should be understood thatthe “active stoichiometric ratio” as used herein is exclusive of anynon-active lithium salts and/or non-active sulfur salts that may beadded to the electrolyte for any purpose, including, e.g., to enhancelithium ion conductivity in the case of e.g., a non-active LiCl salt, ora non-active sulfur containing salt such as, e.g., LiSO₃CF₃.

Accordingly, in embodiments, the catholyte, just prior to initiallyoperating the cell, has an active stoichiometric ratio of Li₂S, Li₂S_(x)(x>1), Li₂S_(x) (1<x<5), Li₂S₅, and Li₂S_(x) (x>5). For example, anactive stoichiometric ratio of about Li₂S, about Li₂S₂, about Li₂S₃,about Li₂S₄, and about Li₂S₅.

In various embodiments, the lithium sulfur cells of the instantinvention include an aqueous catholyte having a high concentration ofdissolved active sulfur species. In embodiments, the sulfurconcentration of active sulfur species in the catholyte is greater than0.5 molar sulfur, greater than 1 molar sulfur, greater than 2 molarsulfur, greater than 3 molar sulfur, greater than 4 molar sulfur,greater than 5 molar sulfur, greater than 6 molar sulfur, greater than 7molar sulfur, greater than 8 molar sulfur, greater than 9 molar sulfur,greater than 10 molar sulfur, greater than 11 molar sulfur, greater than12 molar sulfur, greater than 13 molar sulfur, greater than 14 molarsulfur, greater than 15 molar sulfur, greater than 16 molar sulfur orgreater than 17 molar sulfur.

In particular embodiments, the active lithium sulfur stoichiometricratio in the catholyte just prior to initial cell operation is Li₂S;Li₂S_(x) (x>1); Li₂S_(x) (1<x<5); Li₂S₅; and Li₂S_(x) (x>5), and theconcentration of the dissolved active sulfur species is typicallysignificant, e.g., greater than 1 molar sulfur. For instance, inparticular embodiments, especially for cells using a lithium metal orlithium alloy as the electroactive anode material, the activestoichiometric ratio just prior to initial cell operation is Li₂S_(x)with the following range for x: 2≦x≦5, and the active sulfurconcentration is between 10 to 17 molar sulfur. For example, a catholytecomposition having an active stoichiometric ratio of about Li₂S₄, and atconcentrations greater than 10 molar sulfur (e.g., 11, 12, 13, 14, 15,16 or 17 molar sulfur) may be used. In another particular embodiment,especially useful for cells which are fabricated in the fully or mostlydischarged state (e.g., having an anode electroactive material that isdevoid of active lithium), the active stoichiometric ratio of thecatholyte just prior to initial cell operation is Li₂S, and the activesulfur concentration is typically greater than 1 molar sulfur, andpreferably greater than 2 molar sulfur, and more preferably greater than3 molar sulfur (e.g., 3 molar, 4 molar, or 5 molar sulfur).

Of particular note is the high solubility and facile dissolution of Li₂S(lithium sulfide) in water. In non-aqueous aprotic solvents lithiumsulfide solubility is severely limited, and Li₂S is generally consideredto be insoluble. Water is shown herein to provide an excellent solventfor lithium sulfide (Li₂S), and this feature is used for advantage invarious embodiments of the instant invention in order to achieve highampere-hour (Ah) capacity per unit volume of catholyte, and ultimatelyhigh cell energy density as well as improved reversibility on deepdischarge. A visual comparison is provided in FIG. 5, illustrating thatwater has at least a 1000 fold greater solubility for Li₂S than that oftetraglyne (a common non-aqueous solvent employed in conventionalnon-aqueous Li/S cells).

Accordingly, in various embodiments the aqueous catholyte serves as amedium into which high concentrations of Li₂S dissolve. Thus, by thisexpedient, aqueous lithium sulfur cells yielding a high ampere-hourcapacity per unit volume of catholyte can be realized, and these highcapacity cells may be deeply discharged repeatedly since the reactionproduct (e.g., Li₂S) is readily dissolved and therefore more readilyoxidized on charge. Thus, in various embodiments, at the end ofdischarge a significant portion of the sulfur ampere-hour capacity ispresent in the cell in the form of solid phase discharge product (e.g.,Li₂S). For instance, in embodiments, the end of discharge ratiocomparing the number of moles of sulfur as solid phase sulfur (e.g.,Li₂S) to the number of moles of sulfur dissolved in the catholyte (e.g.,as Li₂S) is greater than 2; greater than 3; greater than 5, or greaterthan 10.

Furthermore, the combination of high solubility and fast dissolutionkinetics of Li₂S in water also enables a practical method of making anaqueous lithium sulfur cell that is assembled in the fully dischargedstate, and which makes use of alternative lithium electroactivematerials that are different than that of lithium metal, such as carbonintercalation materials, alloys (e.g., of silicon) and combinationsthereof such as carbon silicon composites. For example, one method inaccordance with the present invention involves: i) providing a carbonanode in the fully discharged state (i.e., entirely un-intercalated);ii) providing an aqueous polysulfide catholyte comprising water anddissolved lithium sulfide; iii) providing a cathode comprising anelectron transfer medium for electrochemical oxidation of dissolvedlithium sulfide; iv) configuring the anode, catholyte and cathode into abattery cell; and iv) charging the battery cell.

Whereas the fast dissolution kinetics of Li₂S enables repeated deepdischarge, additional benefit may be gained by taking advantage of thefacile electro-kinetics of solution phase redox in combination with thehigh solubility of polysulfide species in water. Thus, in variousembodiments, the cell is formulated such that at full state of chargethe catholyte contains a high concentration of dissolved active sulfurspecies, and in certain embodiments the cell is formulated and operatedsuch that the ampere-hour capacity of sulfur in the cell at full stateof charge is solely present as dissolved species in the catholyte.

Without intending to be limited by theory, lithium sulfide dissolutionin water involves hydrolysis that is believed to take place inaccordance with the following equilibrium:

S²⁻+HOH←→HS⁻+OH⁻

Thus the pH of highly concentrated aqueous catholyte solutions of Li₂Sdissolved in water is generally quite high and typically greater than pH10, and more typically greater than pH 11 or even higher, e.g., about pH12, about pH 13, or about pH 14. However, the invention is notexclusively limited to cells having an aqueous sulfur catholyte of suchhigh pH, as the pH may be tailored using pH adjusting additives,including basic salts (e.g., LiOH), acidic salts (e.g., HCl) andbuffering agents as are known to those of skill in the art.

Thus, in various embodiments the catholyte may be formulated having a pHthat renders it acidic (i.e., pH<7), basic (i.e., pH>7), or neutral (pHabout 7).

The aqueous catholyte may further comprise a supporting lithium salt tomaintain a consistent and high conductivity over the entire dischargeand/or improve stability. Typically the supporting salt concentration isin the range of 0.05 to 1.5 moles/liter (e.g., about 0.25 moles/liter).Examples of suitable supporting salts include a variety of lithiumcation salts. For instance, lithium halides (e.g., LiCl, LiBr),LiSO₃CF₃, LiN(CF₃SO₂)₂ and LiN(SO₂C₂F₅)₂. Typically present in thecatholyte to a concentration of about 0.05 to 1.5 molar lithium, e.g.,0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1.0 molar lithium.

Electroactive aqueous catholytes in accordance with the instantinvention comprise water and an active sulfur species dissolved therein.In various embodiments the active sulfur species are formed in thecatholyte by reacting one or more precursor materials with each otherand/or with water. In one embodiment a first precursor of lithiumsulfide and a second precursor of elemental sulfur are reacted instoichiometric amounts in the presence of water to yield active sulfurspecies in solution. Preferably, to mitigate the undesirable formationof insoluble products of oxidation (e.g., thiosulfates), the watershould be deoxygenated (i.e., the water should be substantially devoidof molecular oxygen), which may be carried out by any suitable methodknown in the art, including boiling of the water and/or purging thewater with an oxygen free gas, such as nitrogen. The purging stepcontinued until the desired level of oxygen has been reached. Forinstance, the concentration of molecular oxygen in the catholyte ispreferably less than 1000 ppm, and more preferably less than 500 ppm andeven more preferably less than 100 ppm, or less than 50 ppm or even 10ppm.

In various embodiments the aqueous catholyte further comprises one ormore non-aqueous solvents. In various embodiments the volume percent ofnon-aqueous solvents in the catholyte ranges from about 1% to as much as90% by volume; for example, between 1% and 10%, between 10% and 20%,between 20% and 30%, between 30% and 40%, between 40% and 50%, between50% and 60%, between 60% and 70%, between 70% and 80%, between 80% and90%.

Non-aqueous solvents suitable for use herein to improve performanceinclude aprotic and protic organic solvents (solids and liquids,typically liquids or solid polyethylene oxide) and ionic liquids. Inparticular, in some embodiments protic organic solvents may be used.

Examples of suitable non-aqueous aprotic and protic solvents includeethers (e.g., 2-Methyltetrahydrofuran (2-MeTHF), Tetrahydrofuran (THF),4-Methyldioxolane (4-MeDIOX), Tetrahydropyran (THP) and 1,3-Dioxolane(DIOX)) glymes (e.g., 1,2-dimethoxyethane (DME/mono-glyme), di-glyme,tri-glyme, tetra-glyme and higher glymes), carbonates (e.g., cycliccarbonates such as propylene carbonate (PC), ethylene carbonate (EC),acyclic carbonates such as dimethyl carbonate (DMC), ethylmethylcarbonate (EMC) and diethyl carbonate (DEC), formates (e.g., MethylFormate) and butyrolactone (GBL). Other suitable aprotic solventsinclude those having a high donor number (i.e., donor solvents) such ashexamethylphosphoramide, pyridine, N,N-diethylacetamide (DMAC),N,N-diethylformamide, dimethylsulfoxide (DMSO), tetramethylurea (TMU),N,N-dimethylacetamide, N,N-dimethylformamide (DMF), tributylphosphate,trimethylphosphate, N,N,N′,N′-tetraethylsulfamide, tetraethylenediamine,tetramethylpropylenediamine, and pentamethyldiethylenetriamine.Preferred donor solvents have a donor number of at least 15, morepreferably between about 15 and 40 and most preferably between about18-40. Particularly preferred donor solvents includeN,N-diethylformamide, N,N-dimethylformamide (DMF), dimethylsulfoxide(DMSO), N,N-dimethylacetamide (DMAC); for example, DMF. Suitableacceptor solvents which can be characterized as Lewis acids (they may beprotic or aprotic solvents) and promote solvation of anions. Examplesinclude alcohols such as methanol, glycols such as ethylene glycol andpolyglycols such as polyethylene glycol as well as nitromethane,triflouroacetic acide, trifluoromethanesulfonic acid, sulfur dioxide andboron triflouride, and ethylene glycol (EG). Others include nitriles(e.g., acetonitrile (AN), higher nitriles, propionitrile,succinonitrile, butyronitrile, benzonitrile), amides (e.g., formamide,N-methylformamide, N,N-dimethylformamide, N,N-diethylformamide, (DMF),acetamide, N-methylacetamide, N,N-dimethylacetamide (DMAC),N,N-diethylacetamide, N,N,N′N′tetraethylsulfamide, tetramethylurea(TMU), 2-pyrrolidone, N-methylpyrrolidone, N-methylpyrrolidinone),amines (e.g., butylamine, ehtylenediamine, triethylamine, pyridine,1,1,3,3-tetramethylguanidine (TMG), tetraethylenediamine,tetramethylpropylenediamine, pentamethyldiethylenetriamine, organosulfursolvents (e.g., dimethylsulfoxide (DMSO), sulfolane, other sulfones,dimethylsulfite, ethylene sulfite, and organophosphorous solvents (e.g.,tributylphosphate, trimethylphosphate, hexamethylphosphoramide (HMPA)).

In the same or separate embodiments a non-aqueous solvent may be addedto the aqueous catholyte to effect dissolution of elemental sulfur. Theaddition of such a solvent (e.g., toluene or carbon disulfide,preferably toluene) can enable charging to elemental sulfur (dissolvedor precipitated).

While the use of non-aqueous solvents such as aprotic organic solvents,typically liquids, but not limited as such, may be useful forfacilitating the dissolution of high order polysulfide species, proticsolvents and ionic liquids may also be incorporated in the aqueouscatholyte to further enhance dissolution of lithium sulfide or moregenerally improve cell performance.

For instance, in particular embodiments the aqueous catholyte compriseswater and a protic solvent that is non-aqueous, especially proticorganic solvents that are capable of dissolving a significant amount ofLi₂S. Particularly suitable non-aqueous protic solvents are organicsolvents such as alcohols, diols, triols and polyols, especiallyalcohols (e.g., methanol and ethanol) and diols (e.g., ethylene glycol).Addition of the non-aqueous protic solvent is particularly useful incells that may be operated at temperatures below the freezingtemperature of water and yet still require high solubility for lithiumsulfide. Accordingly, in various embodiments the catholyte is formulatedwith an amount of a non-aqueous protic solvent to achieve a desiredfreezing point temperature (i.e., melt temperature), includingformulations wherein the melt temperature is less than 0° C., less than−5° C., less than −10° C., less than −15° C., less than −20° C., lessthan −30° C., and less than −40° C. Moreover, it is contemplated hereinthat the non-aqueous protic solvent may be present in high concentrationin the catholyte, including 10%-20%, 20%-30%, 30%-40%, 40%-50%, 50%-60%,60%-70%, 70%-80%, 80%-90% (e.g., such said volume percents of methanol,ethanol or ethylene glycol or combinations thereof.

Contact between the aqueous electrolyte and the cathode electrontransfer medium, for example an electronically conductive matrix such asa carbon or metal mesh, foam or other high surface area, typicallyporous, structure, may be enhanced by electrolyte additives and/orco-solvents. Such improved contact enhances utlilization and rateperformance of the cell. Electrolyte/catholyte compositions in thisregard can include a surfactant, such as a polyol or polyglycol, forexample PEG, to wet the catholyte to the conductive matrix. Also oralternatively, the matrix can be surface treated prior to contact withthe electrolyte to enhance wetting, for example being soaked in awetting agent, such as methanol or ethylene glycol, followed bydisplacement of the wetting agent with the aqueous catholyte solution ofpolysulfides. Still further in this regard, the catholyte may includedissolved organosulfur as a cathode active material. The organosulfurcompound or compounds can self-wet to the cathode electron transfermatrix.

Lithium Anode

Typically, when using a protected lithium electrode as described belowin which a solid electrolyte membrane provides isolation of theelectroactive material against contact with the aqueous catholyte, thecatholyte is devoid of certain extraneous ions which would otherwiseinterfere with the cell functionality, including contaminating themembrane via diffusion into the conductive atomically formed channels.Accordingly, in various embodiments of the instant invention the aqueouscatholyte is substantially devoid of alkali metal cations other thanlithium, and more preferably substantially devoid of all metal cationsother than lithium. For example the catholyte is devoid of sodium andpotassium ions, which is to mean that there is substantially no sodiumor potassium ions in the electrolyte.

The cell comprises a Li anode. The lithium electroactive material of theanode is typically in layered form and may be Li metal or a Li metalalloy (e.g., silicon) or Li intercalation material (e.g., lithiatedcarbon) or in a particular embodiment a silicon carbon composite. In oneexample, a Li metal foil may be used. In another example lithium ionanodes, which are well known in the battery art, are used as theelectroactive lithium material layer (e.g., a carbon intercalationmaterial coated on a copper current collector). Electroactive lithiummaterials, including intercalation host compounds and lithium alloys andlithium metal are well known in the lithium battery art. In certainembodiments the anode is lithium metal (e.g., in foil or sintered form)and of sufficient thickness (i.e., capacity) to enable the cell toachieve the rated discharge capacity of the cell. The anode may take onany suitable form or construct including a green or sintered compact(such as a wafer or pellet), a sheet, film, or foil, and the anode maybe porous or dense. Without limitation, the lithium anode may have acurrent collector (e.g., copper foil, or suitable expandable metal)pressed or otherwise attached to it in order to enhance the passage ofelectrons between it and the leads of the cell. Without limitation thecell may be anode or cathode limited. When anode limited, the completedischarge (corresponding to rated capacity) will substantially exhaustall the lithium in the anode. When cathode limited, some active lithiumwill remain subsequent to the cell delivering its rated capacity.

The anode is protected with a protective membrane architecturechemically stable to both the anode and the environment of the adjacentsulfur cathode. The protective membrane architecture typically comprisesa solid electrolyte protective membrane and an interlayer. The solidelectrolyte protective membrane is sometimes referred to herein as ionmembrane. The protective membrane architecture is in ionic continuitywith the Li anode and is configured to selectively transport Li ionswhile providing an impervious barrier to the environment external to theanode. Protective membrane architectures suitable for use in the presentinvention are described in applicants' U.S. Pat. Nos. 7,645,543;7,666,233; 8,048,571; and 7,282,295, incorporated by reference herein intheir entirely, and in particular for their description of protectivemembrane structures and architectures.

FIGS. 5A-D illustrate representative protective membrane architecturesfrom these disclosures suitable for use in the present invention. Theprotective membrane architectures provide a barrier to isolate a Lianode from ambient and/or the cathode side of the cell while allowingfor efficient ion Li metal ion transport into and out of the anode. Thearchitecture may take on several forms. Generally it comprises a solidelectrolyte layer that is substantially impervious, ionically conductiveand chemically compatible with the external ambient (e.g., air or water)or the cathode environment.

Referring to FIG. 5A, the protective membrane architecture can be amonolithic solid electrolyte 502 that provides ionic transport and ischemically stable to both the active metal anode 501 and the externalenvironment. Examples of such materials are LiHfPO₄, LISICON (thelithium-stable analog to NASICON), Li₅La₃Ta₂O₁₂ and Li₅La₃Nb₂O₁₂,Na₅MSi₄O₁₂ (M: rare earth such as Nd, Dy, Gd) and the garnet-likestructures described below.

More commonly, the ion membrane architecture is a composite composed ofat least two components of different materials having different chemicalcompatibility requirements, one chemically compatible with the anode,the other chemically compatible with the exterior; generally ambient airor water, and/or battery electrolytes/catholytes. By “chemicalcompatibility” (or “chemically compatible”) it is meant that thereferenced material does not react to form a product that is deleteriousto battery cell operation when contacted with one or more otherreferenced battery cell components or manufacturing, handling, storageor external environmental conditions. The properties of different ionicconductors are combined in a composite material that has the desiredproperties of high overall ionic conductivity and chemical stabilitytowards the anode, the cathode and ambient conditions encountered inbattery manufacturing. The composite is capable of protecting an activemetal anode from deleterious reaction with other battery components orambient conditions while providing a high level of ionic conductivity tofacilitate manufacture and/or enhance performance of a battery cell inwhich the composite is incorporated.

Referring to FIG. 5B, the protective membrane architecture can be acomposite solid electrolyte 510 composed of discrete layers, whereby thefirst material layer 512 (also sometimes referred to herein as“interlayer”) is stable to the active metal anode 501 and the secondmaterial layer 514 is stable to the external environment. Alternatively,referring to FIG. 5C, the protective membrane architecture can be acomposite solid electrolyte 520 composed of the same materials, but witha graded transition between the materials rather than discrete layers.

Generally, the solid state composite protective membrane architectures(described with reference to FIGS. 5B and C have a first and secondmaterial layer. The first material layer (or first layer material) ofthe composite is ionically conductive, and chemically compatible with anactive metal electrode material. Chemical compatibility in this aspectof the invention refers both to a material that is chemically stable andtherefore substantially unreactive when contacted with an active metalelectrode material. It may also refer to a material that is chemicallystable with air, to facilitate storage and handling, and reactive whencontacted with an active metal electrode material to produce a productin-situ that is chemically stable against the active metal electrodematerial and has the desirable ionic conductivity (i.e., a first layermaterial). Such a reactive material is sometimes referred to as a“precursor” material. The second material layer of the composite issubstantially impervious, ionically conductive and chemically compatiblewith the first material. Additional layers are possible to achieve theseaims, or otherwise enhance electrode stability or performance. Alllayers of the composite have high ionic conductivity, at least 10⁻⁷S/cm,generally at least 10⁻⁶S/cm, for example at least 10⁻⁵S/cm to 10⁻⁴S/cm,and as high as 10⁻ ³S/cm or higher so that the overall ionicconductivity of the multi-layer protective structure is at least10⁻⁷S/cm and as high as 10⁻³S/cm or higher.

A fourth suitable protective membrane architecture is illustrated inFIG. 5D. This architecture is a composite 530 composed of an interlayer532 between the solid electrolyte 534 and the active metal anode 501whereby the interlayer is includes a non-aqueous liquid, gel or solidpolymer electrolyte polymer phase anolyte. Thus, the architectureincludes an active metal ion conducting separator layer with anon-aqueous anolyte (i.e., electrolyte in contact with the anodeelectroactive material), the separator layer being chemically compatiblewith the active metal and in contact with the anode; and a solidelectrolyte layer that is substantially impervious (pinhole- andcrack-free) ionically conductive layer chemically compatible with theseparator layer and aqueous environments and in contact with theseparator layer. The solid electrolyte layer of this architecture (FIG.5D) generally shares the properties of the second material layer for thecomposite solid state architectures (FIGS. 5B and C). Accordingly, thesolid electrolyte layer of all three of these architectures will bereferred to below as a second material layer or second layer.

A wide variety of materials may be used in fabricating protectivecomposites in accordance with the present invention, consistent with theprinciples described above. For example, in the solid state embodimentsof FIGS. 5B and 5C, the first layer (material component), in contactwith the active metal, may be composed, in whole or in part, of activemetal nitrides, active metal phosphides, active metal halides activemetal sulfides, active metal phosphorous sulfides, or active metalphosphorus oxynitride-based glass. Specific examples include Li₃N, Li₃P,LiI, LiBr, LiCl, LiF, Li₂S—P₂S₅, Li₂S—P₂S₅—LiI and LiPON. Active metalelectrode materials (e.g., lithium) may be applied to these materials,or they may be formed as reaction products in situ by contactingprecursors such as metal nitrides, metal phosphides, metal halides, redphosphorus, iodine, nitrogen or phosphorus containing organics andpolymers, and the like with lithium. A particularly suitable precursormaterial is copper nitride (e.g., Cu₃N). The in situ formation of thefirst layer may result from an incomplete conversion of the precursorsto their lithiated analog. Nevertheless, such composite reactionproducts formed by incomplete conversions meet the requirements of afirst layer material for a protective composite in accordance with thepresent invention and are therefore within the scope of the invention.

For the anolyte interlayer composite protective architecture embodiment(FIG. 5D), the protective membrane architecture has an active metal ionconducting separator layer chemically compatible with the active metalof the anode and in contact with the anode, the separator layercomprising a non-aqueous anolyte, and a substantially impervious,ionically conductive layer (“second” layer) in contact with theseparator layer, and chemically compatible with the separator layer andwith the exterior of the anode. The separator layer can be composed of asemi-permeable membrane impregnated with an organic anolyte. Forexample, the semi-permeable membrane may be a micro-porous polymer, suchas are available from Celgard, Inc. The organic anolyte may be in theliquid or gel phase. For example, the anolyte may include a solventselected from the group consisting of organic carbonates, ethers,lactones, sulfones, etc, and combinations thereof, such as EC, PC, DEC,DMC, EMC, 1,2-DME or higher glymes, THF, 2MeTHF, sulfolane, andcombinations thereof. 1,3-dioxolane may also be used as an anolytesolvent, particularly but not necessarily when used to enhance thesafety of a cell incorporating the structure. When the anolyte is in thegel phase, gelling agents such as polyvinylidine fluoride (PVdF)compounds, hexafluropropylene-vinylidene fluoride copolymers (PVdf-HFP),polyacrylonitrile compounds, cross-linked polyether compounds,polyalkylene oxide compounds, polyethylene oxide compounds, andcombinations and the like may be added to gel the solvents. Suitableanolytes will, of course, also include active metal salts, such as, inthe case of lithium, for example, LiPF₆, LiBF₄, LiAsF₆, LiSO₃CF₃ orLiN(SO₂C₂F₅)₂. One example of a suitable separator layer is 1 M LiPF₆dissolved in propylene carbonate and impregnated in a Celgardmicroporous polymer membrane.

The second layer (material component) of the protective composite may becomposed of a material that is substantially impervious, ionicallyconductive and chemically compatible with the first material orprecursor, including glassy or amorphous metal ion conductors, such as aphosphorus-based glass, oxide-based glass, phosphorus-oxynitride-basedglass, sulfur-based glass, oxide/sulfide based glass, selenide basedglass, gallium based glass, germanium-based glass, Nasiglass; ceramicactive metal ion conductors, such as lithium beta-alumina, sodiumbeta-alumina, Li superionic conductor (LISICON), and the like; orglass-ceramic active metal ion conductors. Specific examples includeLiPON, Li₃PO₄.Li₂S.Si₂, Li₂S.GeS₂.Ga₂S₃, Li₂O.11Al₂O₃, Na₂O.11Al₂O₃,Li_(1+x)Ti_(2−x)Al_(x)(PO₄)₃ (0.1≦x≦0.9) and crystallographicallyrelated structures, Li_(1+x)Hf_(2−x)Al_(x)(PO₄)₃ (0.1≦x≦0.9),Li₃Zr₂Si₂PO₁₂, Na₅ZrP₃O₁₂, Li-Silicates, Li_(0.3)La_(0.5)TiO₃,Li₅MSi₄O₁₂ (M: rare earth such as Nd, Gd, Dy) Li₅ZrP₃O₁₂, Li₅TiP₃O₁₂,Li₃Fe₂P₃O₁₂ and Li₄NbP₃O₁₂, and combinations thereof, optionallysintered or melted. Suitable ceramic ion active metal ion conductors aredescribed, for example, in U.S. Pat. No. 4,985,317 to Adachi et al.,incorporated by reference herein in its entirety and for all purposes.

A particularly suitable glass-ceramic material for the second layer ofthe protective composite is a lithium ion conductive glass-ceramichaving the following composition:

Composition mol % P₂O₅ 26-55%  SiO₂ 0-15% GeO₂ + TiO₂ 25-50%  in whichGeO₂ 0-50% TiO₂ 0-50% ZrO₂ 0-10% M₂O₃ 0-10% Al₂O₃ 0-15% Ga₂O₃ 0-15% Li₂O3-25%

and containing a predominant crystalline phase composed ofLi_(1+x)(M,Al,Ga)_(x)(Ge_(1−y)Ti_(y))_(2−x)(PO₄)₃ where X≦0.8 and0≦Y≦1.0, and where M is an element selected from the group consisting ofNd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb and/orLi_(1+x+y)Q_(x)Ti_(2−x)Si_(y)P_(3−y)O12 where 0<X≦0.4 and 0≦Y≦0.6, andwhere Q is Al or Ga. The glass-ceramics are obtained by melting rawmaterials to a melt, casting the melt to a glass and subjecting theglass to a heat treatment. Such materials are available from OHARACorporation, Japan and are further described in U.S. Pat. Nos.5,702,995, 6,030,909, 6,315,881 and 6,485,622, incorporated herein byreference.

Another particularly suitable material for the second layer of theprotective composite is a lithium ion conducting oxide having a garnetlike structure. These include Li₆BaLa₂Ta₂O₁₂; Li₇La₃Zr₂O₁₂,Li₅La₃Nb₂O₁₂, Li₅La₃M₂O₁₂ (M=Nb, Ta)Li_(7+x)A_(x)La_(3−x)Zr₂O₁₂ where Amay be Zn. These materials and methods for making them are described inU.S. Patent Application Pub. No.: 2007/0148533 (application Ser. No:10/591,714), hereby incorporated by reference in its entirety, andsuitable garnet like structures are also described in InternationalPatent Application Pub. No.: WO/2009/003695 which is hereby incorporatedby reference for all that it contains, and in particular for itsdescription of garnet-like structures.

The composite should have an inherently high ionic conductivity. Ingeneral, the ionic conductivity of the composite is at least 10⁻⁷ S/cm,generally at least about 10⁻⁶ to 10⁻⁵ S/cm, and may be as high as 10⁻⁴to 10⁻³ S/cm or higher. The thickness of the first precursor materiallayer should be enough to prevent contact between the second materiallayer and adjacent materials or layers, in particular, the active metalof the anode. For example, the first material layer for the solid statemembranes can have a thickness of about 0.1 to 5 microns; 0.2 to 1micron; or about 0.25 micron. Suitable thickness for the anolyteinterlayer of the fourth embodiment range from 5 microns to 50 microns,for example a typical thickness of Celgard is 25 microns.

The thickness of the second material layer is preferably about 0.1 to1000 microns, or, where the ionic conductivity of the second materiallayer is about 10⁻⁷ S/cm, about 0.25 to 1 micron, or, where the ionicconductivity of the second material layer is between about 10⁻⁴ about10⁻³ S/cm, about 10 to 1000 microns, preferably between 1 and 500microns, and more preferably between 10 and 100 microns, for exampleabout 20 microns.

Seals and methods of making seals which are particularly suitable forsealing protected anodes described hereinabove and elsewhere, includingcompliant and rigid seals, are fully described in U.S. PatentPublication No.: 2007/0037058 and U.S. Patent Publication No.: U.S.2007/0051620 to Visco et al., and are hereby incorporated by referencein their entirety, and in particular for their descriptions of cellseals and sealing techniques.

Optional Separator

With reference to FIG. 1 an optional separator component 130 may beinterposed between the membrane architecture and the sulfur cathode.Various separator materials suitable for use herein are known in thebattery arts. These separators include porous inorganic mats,microporous polymer sheets, and gels. In a particular embodiment theseparator is a hydrogel comprising water impregnated a polymer. In someembodiments the polymer itself may also serve as a solid solvent for thedissolution of active sulfur species, such as PEO and polyalcohols(e.g., polyvinyl alcohol).

In various embodiments the instant battery cell is fabricated such thatthe entirety of the cathode capacity is loaded into the cell uponfabrication as dissolved polysulfide species (e.g., the activestoichiometric ratio of Li₂Sx with x is >1 e.g., about Li₂S₂, aboutLi₂S₃, about Li₂S₄, and about Li₂S₅). In certain embodiments solid phasesulfur is added to further enhance cell capacity (i.e., the cathodeactive species derived from a combination of dissolved polysulfidespecies and solid elemental sulfur. In some embodiments the entirety ofthe cathode active sulfur is loaded into the cathode as solid elementalsulfur. While in other embodiments, as described herein, the catholyteis in a fully reduced state composed of Li₂S dissolved in water, and insome embodiments thereof solid phase Li₂S may be dispersed in thecatholyte or present as a solid particle in the pores of the cathode orseparator.

In accordance with various embodiments of the instant invention asignificant amount of the cathode ampere-hour capacity is derived fromthe active aqueous sulfur catholyte, and that amount is typicallygreater than 10%; for instance, greater than 20%, greater than 30%,greater than 40%, greater than 50%, greater than 60%, greater than 70%,greater than 80%, greater than 90%, and in certain embodiments 100%.

Flow Cells

With reference to FIG. 6 there is illustrated a representativeembodiment of an aqueous lithium sulfur flow cell battery system 600 inaccordance with the instant invention. The system includes a reactorcell 660 in which there is positioned a lithium anode 120 and a sulfurcathode 110 configured, in one embodiment, in a spatially apartrelationship, therewith defining an inter-electrode region 650 throughwhich an aqueous sulfur catholyte is caused to flow during operation. Invarious embodiments the lithium anode is a protected lithium electrodeas described above and the sulfur cathode likewise as described above.In a slightly modified embodiment the sulfur cathode, a porous threedimensional body, is positioned in direct contact with the first surfaceof the protected anode solid electrolyte membrane architecture (i.e.,not in a spatially apart relationship) and the aqueous catholyte iscaused to flow into the pores of the cathode structure.

Continuing with reference to FIG. 6 the system further comprises anexternal reservoir system, which may take the form of a storage tank 620for storing the aqueous sulfur catholyte to be flowed through theinter-electrode region or channel. The reservoir system may also includepipeworks 610 for fluidly coupling the tank to the reactor, and a pump603 for circulating the electrolyte through the channel. The pipeworksmay have valves (not shown) for closing or opening the reactor cell tothe storage tank. The pump may be operated for circulating theelectrolyte through the channel, and the valves may be used to controlthe flow of catholyte through the reactor.

The aqueous catholyte provides the electroactive sulfur species, whichare electrochemically reacted at the sulfur electrode during charge anddischarge. In operation, the aqueous catholyte from the storage tank iscaused to flow by or through the sulfur cathode, and dissolvedpolysulfide species are electro-reduced when the system is deliveringelectricity (during discharge) and electro-oxidized when storingelectricity on charge.

Since the ampere-hour capacity of the cathode is provided by the aqueouscatholyte in the storage tank, the sulfur cathode is typically assembledin the reactor cell devoid of elemental sulfur. For instance, the sulfurcathode may be a carbon matrix optionally coated with a catalyst tofacilitate polysulfide redox while inhibiting hydrogen evolution.Moreover, during system assembly, while the lithium electroactivematerial of the anode may be incorporated in a fully charged state(e.g., in the form of a lithium metal foil), in preferred embodiments itis an intercalation material or alloy material that is incorporated inthe fully discharged state (i.e., devoid of any active lithium). Carbonmaterials such as graphitic carbons capable of reversibly intercalatinglithium are a particularly suitable lithium electroactive material foruse in the instant flow cell system. Others include lithium alloyingmaterials, as described above, such as silicon and tin which are capableof reversibly absorbing/desorbing lithium electrochemically, as well ascomposite carbon silicon materials.

Held in the storage tank, the aqueous catholyte effectively provides thecathode fuel for the electrochemical reaction at the sulfur cathode, andthe aqueous catholytes embodiments described above with reference to thebattery cell embodiment illustrated in FIG. 1 are suitable for useherein as a cathode fuel. The aqueous catholyte fuel comprisespolysulfide species dissolved in water. In embodiments the concentrationof the dissolved polysulfide species in the electrolyte is in the rangeof 0.5 to 1 molar sulfur, 1 to 2 molar sulfur, 2 to 3 molar sulfur, 3 to4 molar sulfur, 4 to 5 molar sulfur, 5 to 6 molar sulfur, 6 to 7 molarsulfur, 7 to 8 molar sulfur, 8 to 9 molar sulfur, 9 to 10 molar sulfur,and in some embodiments the concentration of polysulfide species isgreater than 10 molar sulfur, greater than 11 molar, greater than 12molar, greater than 13 molar, greater than 14 molar, greater than 15molar, and greater than 16 molar.

In one embodiment the system is assembled with the lithium electroactivematerial in the discharged state (e.g., carbon intercalation materialdevoid of intercalated lithium), and the aqueous catholyte comprisinghighly reduced polysulfide species, e.g., dissolved Li₂S. For example,the aqueous catholyte a solution of about 3 molar Li₂S dissolved inwater. Aqueous sulfur catholyte storage tanks having enhanced sulfurcapacity (i.e., greater sulfur capacity per unit volume) may be achievedby adding additional solid lithium sulfide to the catholyte beyond itssolubility limit (i.e., a saturated water solution of Li₂S). Because ofthe fast kinetics of lithium sulfide dissolution in water, additionalcatholyte capacity may be added to the tank by dispersing or suspendingsolid phase lithium sulfide in the aqueous catholyte.

Continuing with reference to the above embodiment, the system isassembled in the fully discharged state so it must undergo an initialcharge reaction to lithiate the carbon intercalation material. Theinitial charge may be conducted via electro-oxidation of the reducedaqueous catholyte (e.g., 3 molar Li₂S water solution) or a conditioningcatholyte formulation may be used, for instance one in which sulfur isnot the electroactive species. For example, the initial charge may becompleted by using a water based lithium nitrate catholyte solution thatis circulated or caused to flow past the cathode, whereupon the water iselectro-oxidized and oxygen evolved, while at the anode lithium ionsfrom the conditioning catholyte electro-reductively intercalate into thecarbon. The conditioning catholyte flowing through the channel may beelectro-oxidized until the reaction is complete and the carbon issufficiently or fully lithiated. Thereafter, the conditioning catholytetank is replaced by a tank of aqueous sulfur catholyte.

In embodiments wherein the lithium electroactive material is fully ormostly charged via the lithiation step described above (e.g., by using aconditioning catholyte), the aqueous catholyte may then be formulated ina highly oxidative state; for instance, as elemental sulfur dispersed orsuspended in a water solution typically also comprising a dissolvedlithium salt (e.g., lithium hydroxide) to support the ionic current. Itis contemplated that toluene may be added to the catholyte in order todissolved some of the dispersed solid sulfur and by this expedientfacilitate electro-reduction at the sulfur cathode.

Various compositions of the as formulated catholyte storage tanks arecontemplated. In various embodiments the flow cell is operated such thatthe active stoichiometric lithium sulfur ratio is Li₂S_(x) with (1<x<5),(x=5), or (x>5)

In the aforementioned flow cell embodiments, the lithium electroactivematerial is stationary, which is to mean that it is non-flowing andincorporated as a component of the protected lithium electrode, e.g.,typically in the form of a layer such as a sintered layer or a coatingon a current collector as is well known in the field of lithium ionbatteries. Thus, the capacity of the anode is set once the coating isformed and the system is assembled.

In an alternative embodiment, with reference to the flow cell system 700illustrated in FIG. 7, the structure of FIG. 6 is supplemented by areactor cell 760 configured for through flow of a flowable lithiumelectroactive material (e.g., an electroactive lithium slurry) betweenan anode current collector 122 on which the electrochemical reactionstake place and the second surface of a substantially impervious lithiumion conducting membrane architecture 126. Flowable lithium electroactivematerials suitable for use herein are described in U.S. PatentApplication Pub. Nos.: 2011/0200848 of Chiang et al., published Aug. 18,2011 and 2010/0323264 of Chiang et al., published Dec. 23, 2011, andeach of these is hereby incorporated by reference for all that theycontain in this regard. Generally these are anode particles dispersed inan ionically conductive carrier fluid that is compatible with the anodeparticles over the range of oxidation state encompassing full charge tofull discharge. Particularly suitable anode particulates areintercalation carbons or alloy materials such as silicon, or acombination of these (e.g., carbon-silicon composite). The anode currentcollector 122 is disposed in the cell in spaced relation to theprotective membrane architecture, thus defining a channel 702 throughwhich the lithium electroactive slurry is caused to flow, for instancevia pumping action. The flow system includes a second external reservoirsystem for the lithium anode, which may take the form of a storage tank720B for storing the lithium anode slurry and pipeworks 710B for fluidlycoupling the tank to the reactor cell, and a pump 703B for circulatingthe slurry through the channel, similar to that which is described abovefor circulating the sulfur catholyte.

EXAMPLES

Aqueous Catholytes with Dissolved Active Sulfur Species

The following examples provide details illustrating the preparation andadvantageous properties, including high ionic conductivity, of aqueouscatholytes with dissolved active sulfur species suitable for use inelectrochemical cells in accordance with the present invention. Theseexamples are provided to exemplify and more clearly illustrate aspectsof the present invention and are in no way intended to be limiting.

Example 1

This example pertains to the preparation and conductivity measurement ofa first active aqueous sulfur catholyte (i.e., Catholyte #1) havingwater as a solvent, an active stoichiometric ratio of Li₂S₄, and asulfur concentration of 10 moles/liter (molar) sulfur. The precursorchemicals Li₂S and elemental sulfur are used in proper proportion toyield an active stoichiometric ratio of Li₂S₄. In addition to theprecursor chemicals, Catholyte #1 also contains, dissolved therein, anadditional basic lithium salt, specifically 0.5 molar LiOH.

The catholyte was prepared in a 25 mL volumetric flask inside a mainglove box filled with argon gas (i.e., an inert gas), the glove boxhaving oxygen concentration of less than 5 ppm (i.e., the environment inwhich the catholyte is made is substantially devoid of molecularoxygen). The required amount of lithium hydroxide (reagent grade, SigmaAldrich) was weighed in a different glove box filled with dry argonhaving less than 2 ppm of moisture and then was transferred to the mainglove box used for catholyte preparation. Deionized water was boiled andtransferred to the same glove box in a closed container. Inside theglove box, argon gas was bubbled through the container in order toremove remaining traces of oxygen. The required amount of Li₂S (SigmaAldrich, 99% purity) was determined from the reaction 8 Li₂S+3 S₈→8Li₂S₄ and mixed with the lithium hydroxide, and placed in the flask.Next, 10 mL of the deionized and deoxygenated water was added to theflask, and the mixture was stirred for 30 minutes. Based on theaforementioned stoichiometric reaction and Li₂S₄ as the desired activestoichiometric ratio, the required amount of sulfur (Sigma Aldrich,reagent grade, purified by sublimation) was added to the mixture. Then,deionized water was added to the mixture up to the 25 mL mark, the flaskwas tightly sealed (to avoid active sulfur loss in the form of H₂S gas),and the mixture was stirred overnight. Next day, the stir bar wasremoved, water was added up to the 25 mL mark, and the solution wasstirred for another hour. The obtained solution was reddish orange anddid not contain any visible solids. Conductivity of the preparedcatholyte was measured using a conductometric cell (RadiometerAnalytical S.A., France) with two platinized platinum electrodes. Theobtained specific conductivity value is high (i.e., greater than 10⁻²S/cm) and specifically the measured value was 0.1 S/cm.

Example 2

This example pertains to the preparation and conductivity measurement ofa second active aqueous sulfur catholyte (i.e., Catholyte #2) havingwater as a solvent, an active stoichiometric ratio of Li₂S₄, and asulfur concentration of 12 moles/liter (molar) sulfur. Similar to theprocedure described in Example #1, the precursor chemicals Li₂S andelemental sulfur were used to effect the active Li₂S₄ activestoichiometric ratio. The catholyte was devoid of salts (e.g.,additional lithium salts) other than those used to generate the activestoichiometric ratio of Li₂S₄. In particular, the catholyte was devoidof supporting lithium salts or basic lithium salts.

The catholyte was prepared in a manner similar to that for Catholyte #1,as described above in Example 1. Required amounts of the precursorchemicals (sulfur and Li₂S) were mixed together, placed in a 25 mLvolumetric flask, and covered with deionized and deoxygenated water upto the 25 mL mark. Notably the water is deoxygenated prior to contactingthe precursor chemicals. The flask was tightly sealed and the contentswere stirred. The mixture quickly turned reddish orange and itstemperature rose significantly. Dissolution (via, in part, hydrolysis)occurred quickly, and much faster than during preparation of Catholyte#1 since the presence of LiOH slows down the rate of Li₂S hydrolysis.After stirring the mixture overnight, a clear reddish orange liquid wasobtained. Thereafter the stir bar was removed, water was added up to the25 mL mark, and the solution was stirred for another hour. Theconductivity of the prepared catholyte was measured in a manner similarto that described in Example 1, and a specific conductivity value of8×10⁻² S/cm was obtained.

Example 3

This example pertains to the preparation of a third active aqueoussulfur catholyte (i.e., Catholyte #3) having water as a solvent, anactive stoichiometric ratio of Li₂S₄, and a sulfur concentration of 17moles/liter (molar) sulfur. Similar to that described in Example #1, theprecursor chemicals Li₂S and elemental sulfur are used to effect theactive Li₂S₄ active stoichiometric ratio. The catholyte is devoid ofsalts (e.g., additional lithium salts) other than those used to generatethe active stoichiometric ratio of Li₂S₄. In particular, the catholyteis devoid of supporting lithium salts or basic lithium salts.

In order to prepare a catholyte with the highest possible active sulfurcontent (in the form of Li₂S₄), enough sulfur and Li₂S were mixed toprepare 20M sulfur having an active stoichiometric ratio of Li₂S₄. Thenthe same procedure as used in Example 2 was followed. After stirringovernight, the solution was not clear and contained undissolved solids.The solution was filtered through a glass microfiber GF/A filter and theclear filtrate (i.e., clear catholyte solution) was analyzed for totaldissolved sulfur content using a method that was described in thearticle by G. Schwarzenbach, A. Fischer in Heir. Chim. Acta 43,1365-1390 (1960), and entitled Die Acidität der sulfane and diezusammensetzung wässeriger polysulfdlosungen. The article, andspecifically the method for determining sulfur concentration, is herebyincorporated by reference. In particular, the dissolvedsulfur-containing species were oxidized to sulfate, which was thentitrated by barium perchlorate in the presence of Thorin indicator. Thedetermined sulfur concentration in the catholyte (i.e., sulfur molarity)was 17.25M sulfur (i.e., greater than 17 molar sulfur).

Example 4

This example pertains to the preparation and conductivity measurement ofa fourth active aqueous sulfur catholyte (i.e., Catholyte #4) havingwater as a solvent, an active stoichiometric ratio of Li₂S, and a sulfurconcentration of 3 moles/liter sulfur (3 molar sulfur). In this example,the precursor chemical was solely Li₂S, and the catholyte was devoid ofadditional salts (e.g., additional lithium salts). In particular, thecatholyte was devoid of supporting lithium salts or basic lithium salts.

The required amount of Li₂S was placed in a volumetric flask anddeionized and deoxygenated water (as described above) was added to the25 mL mark. The mixture quickly turned reddish orange and itstemperature rose significantly. The mixture was stirred overnight, thenthe stir bar was removed, water was added up to the 25 mL mark, and thesolution was stirred for another hour. The resulting liquid was clearand had a reddish orange color. This experiment indicates that thesolubility of Li₂S in water is quite high.

The conductivity of the prepared catholyte containing products of Li₂Shydrolysis was measured in a manner similar to that used in Example 1,and an exceptionally high value of 2×10⁻¹ S/cm was obtained (i.e.,greater than 10⁻¹ S/cm).

By this expedient, and as described herein below in Example #11, waterdissolved Li₂S may be used as a source of active Li for insertion (e.g.,intercalation), for instance, for the purpose of charging alternativeanodes such as carbon-based intercalation materials and other materialsthat are devoid of active lithium upon cell fabrication, including thoseinstances in which the cell is assembled in a discharged state (e.g., afully discharged state). Moreover, the high solubility and fastdissolution kinetics of Li₂S in water eliminates or significantlyreduces problems associated with precipitation of Li₂S discharge producton the cathode surface (or inside the cathode pore space) where it canadversely effect cell performance, especially cycle life.

Example 5

This example pertains to the preparation and conductivity measurement ofa protic non-aqueous active sulfur catholyte (i.e., Catholyte #5) havingalcohol as a solvent (specifically methanol), an active stoichiometricratio of Li₂S₄, and a sulfur concentration of 6 moles/liter (molar)sulfur. Similar to the procedure described in Example #1, the precursorchemicals Li₂S and elemental sulfur were used to effect the active Li₂S₄active stoichiometric ratio. The catholyte was devoid of salts (e.g.,additional lithium salts) other than those used to generate the activestoichiometric ratio of Li₂S₄. In particular, the catholyte was devoidof supporting lithium salts or basic lithium salts.

The required amount of sulfur and Li₂S precursor chemicals were placedin a 25 mL volumetric flask, and the rest of the operations were similarto those described in Example #2, except that methanol was used insteadof water. The resulting protic non-aqueous catholyte was clear and had areddish orange color. Its conductivity was measured to be 1.1×10⁻² S/cm.

Electrochemical Testing of Li/S Cells

The following examples provide details illustrating electrochemicaltesting of Li/S cells in accordance with the present invention. Theseexamples are provided to exemplify and more clearly illustrate aspectsof the present invention and are in no way intended to be limiting.

Preparation of Cathode Materials:

Carbon based electron transfer mediums were used as the cathode (i.e.,carbon based cathodes). Specifically, a porous carbon paper matrix(Lydall Technical Papers, Rochester, N.Y.) coated with a carbon binderslurry of 70 (wt) % acetylene black and 30% PVdF, with a dry slurryweight of about 1.3 mg/cm² was used.

Lead based electron transfer mediums used as the cathode (i.e., leadbased cathodes) were prepared by electroplating lead as a surfacecoating onto a core substrate of nickel (Ni ExMet type 5Ni 5-050 fromDEXMET Corp.). The lead was coated from a solution having the followingcomposition:

-   200 g/L Lead (II) Carbonate, PbCO₃,-   100 mL/L Tetrafluoroboric acid, HBF₄,-   15 g/L Boric acid, H₃BO₃,-   5 g/L Hydroquinone.

A rectangular piece of lead foil with a thickness of 1.6 mm was used asan anode during electroplating. The current density was 5 mA/cm² and thethickness of the deposited lead coating was approximately 30 μm.

Cobalt based electron transfer mediums used as the cathode (i.e., cobaltbased cathodes) were prepared by electroplating cobalt onto a coppersubstrate (Cu ExMet 1.5 Cu 5.5-OSOF1 from Delker Corp.) from a solutionhaving the following composition:

-   450 g/L Cobalt Sulfate Heptahydrate, CoSO₄.7H₂O,-   15 g/L Sodium Chloride, NaCl,-   40 g/L Boric acid, H₃BO₃.

A graphite plate with a thickness of 6 mm served as an anode duringelectroplating. Electroplating was performed at a temperature of 35-40°C. at a current density of 20 mA/cm² and the resulting thickness of thedeposited cobalt was approximately 25 μm.

Example 6 Determination of Potential Window for Li/S Cell OperationUsing Cyclic Voltammetry

Cyclic voltammetry experiments were performed in hermetically sealedglass cells with plastic covers. The cells were assembled and filledwith polysulfide-containing aqueous electrolyte in a glove boxcontaining argon gas with an oxygen concentration of less than 5 ppm(i.e., substantially devoid of molecular oxygen). Aqueous electrolytecontaining polysulfides had a composition of 4M Sulfur and an activestoichiometric ratio of Li₂S₄. For comparison testing, an aqueouselectrolyte based on lithium sulfate, which did not contain activesulfur species, was also prepared. The pH of the second electrolyte wasadjusted to the pH of the first electrolyte (pH 12) by addition of LiOH.

The working electrode was either a 1 cm×1 cm square carbon paper-basedcathode or a 1 cm×1 cm square lead cathode as described above. Theworking electrode was located between two protected lithium electrodes(as described herein above) serving as counter-electrodes in the cell.Lithium foil area was 22 mm×22 mm in each of the counter electrodes.Working electrode potential was measured vs. an Ag/AgCl referenceelectrode and then was recalculated into potentials vs. a Li/Li⁺electrode. The cyclic voltammetry curves were measured using a VMP-3potentiostat/galvanostat (Bio-Logic Science Instruments, France) at ascan rate of 0.5 mV/s.

FIG. 8 shows cyclic voltammetry curves for a carbon electrode in aqueouselectrolytes with and without dissolved polysulfides. The cyclicvoltammetry curves have several characteristic regions. (Region A ismagnified on the right graph of FIG. 8). The voltammetry curve of thesulfate electrolyte allows determination of the hydrogen evolutionpotential (cathodic current in region A at potentials below 2.0V) andoxygen evolution (anodic current in region D at potentials above 3.8V)on the surface of the carbon electrode (i.e., carbon based electrontransfer medium). Comparison of voltammetry curves for the twoelectrolytes indicates that cathodic currents in region A for thepolysulfide electrolyte are attributed to electroreduction ofsulfur-containing species. The right graph clearly shows that in orderto minimize the contribution of the side reaction (hydrogen evolution)in the cell with a carbon based electron transfer medium serving ascathode, the cell discharge voltage should not be allowed to go belowapproximately 2.0V in certain embodiments. Region B on the polysulfideelectrolyte curve corresponds to the electrooxidation ofsulfur-containing species. Highly oxidized sulfur-containing species candecompose forming elemental sulfur, which can also be formed directly athigh enough positive potentials. Deposition of insulative sulfur on thecarbon surface leads to a decrease in current (region C) and largehysteresis on the cyclic voltammetry curve at potentials over 2.7-2.8V.

FIG. 9 demonstrates that the lead based electrode has a significantlygreater overpotential for hydrogen evolution than the carbon electrode.Therefore, the use of lead on the electron transfer medium allows for anincrease in the potential window for Li/S cell operation.

FIG. 10 shows cyclic voltammetry curves in a wide potential range forcarbon and lead positive electrodes (i.e., cathodes) in electrolytescontaining dissolved polysulfides. These curves demonstrate that theprepared lead electrode had a better rate capability than the carbonelectrode.

Example 7 Cyclic Performance of Li/S Cells With Carbon Cathode

Cyclability tests were performed in hermetically sealed Li/S cellshaving two compartments: a protected lithium anode compartment and anaqueous sulfur cathode compartment. A substantially imperviousglass-ceramic membrane, as described herein above, was fitted into thecell by means of two Kalrez o-rings such that the membrane was exposedto the aqueous catholyte from the cathode side and to the non-aqueouselectrolyte from the anode side. The anode compartment was assembled inan argon-filled dry box and contained a 125 μm-thick lithium foil fromFMC Lithium Corp in a shape of a disc with a diameter of ½″ pressed ontoa nickel foil current collector, a 1″×1″ square 150 μm-thickglass-ceramic solid electrolyte membrane from Ohara Corp. (Japan), andCelgard 2400 microporous separator in a shape of a disc with a diameterof 9/16″. The separator was impregnated with a non-aqueous electrolytecontaining 1 M of LiTFSI salt in 1,3-dioxolane and placed between the Lifoil surface and the glass-ceramic membrane.

After the anode compartment was built, it was transferred to the dry boxfilled with oxygen-free argon, where the cathode compartment wasassembled, filled with aqueous catholyte and hermetically sealed. Theaqueous catholyte (Catholyte #2) contained 12M S as Li₂S₄ in water. A9/16″-diameter disc of microporous Celgard 3401 separator wasimpregnated with the catholyte and placed on the surface of theglass-ceramic protective membrane. The carbon cathode described abovewas cut in a shape of a ½″-diameter disc and placed on top of theCelgard 3401 separator layer. A ½″-diameter stainless steel disc wasused as a cathode current collector. The components of the cathodecompartment were kept in contact with a stainless steel spring. Theassembled cell exhibited an open circuit voltage of greater than 2.5volts.

Cell cycling was performed using a Maccor battery tester. The cyclingprocedure was as follows. The first discharge at a current density of 1mA/cm² to the cut-off voltage of 2.1V was followed by a charge at 0.5mA/cm² to the capacity equal to the previous discharge capacity. Thesecond discharge was equal to the previous charge capacity. Then thecell was cycled at a constant capacity corresponding to the seconddischarge. The charge cut-off voltage was set to 2.8V.

FIG. 11 shows the cycling performance of the Li/S cell. The cellexhibited good cyclability and over 100 cycles were achieved. This isthe first known example of a rechargeable aqueous Li/S cell havingdissolved active sulfur species.

FIG. 12 shows charge and discharge voltage profiles. A high round-tripefficiency value of 87% was calculated from average discharge and chargevoltages.

Example 8

The Li/S cell and catholyte composition were the same as described inExample #7. However, in this case the carbon based cathode and thestainless steel cathode current collector were immersed overnight in asolution with the same composition as the Li/S cell catholyte, 12M Shaving an active stoichiometric ratio of Li₂S₄ in water. The goal ofthis pre-treatment was to avoid consumption of active sulfur species bythe reaction with the cathode and the current collector in the assembledcell. After storage in the catholyte solution overnight, the cathode andthe current collector were removed and rinsed in sequence with 0.5MLiOH, water, toluene, and methanol, and then dried. It was found thatthe pre-treatment in a sulfur-containing solution greatly improved thewettability of the carbon electrode with catholyte during cathodecompartment filling. The cycling procedure included a discharge at acurrent density of 1 mA/cm² to the cut-off voltage of 2.0V and a chargeat 0.5 mA/cm² to the capacity equal to the previous discharge capacity.The charge cut-off voltage was set to 2.8V.

Voltage-time discharge/charge profiles and delivered capacity vs. cyclenumber plots are shown in FIG. 13. Under described test conditions, thecell demonstrated good cycle life of over 50 cycles with small capacityfade.

Example 9

The cell and catholyte composition and cycling procedure were the sameas described in Example #7. However, instead of a carbon electrode witha nickel current collector, a lead electrode with a lead currentcollector was used. The electrode and the current collector werepre-treated in the catholyte solution as described in Example #8.

As seen in FIG. 14, which shows voltage-time discharge/charge profilesand delivered capacity vs. cycle number plots, Li/S cells using the leadbased cathode can be cycled at a high areal capacity of approximately 12mAh/cm².

Example 10

The cell and catholyte composition were the same as described in Example#7. The cycling procedure was the same as described in Example #8.However, instead of a carbon electrode with a nickel current collector,a cobalt electrode described above with a cobalt-electroplated coppercurrent collector was used. The electrode and the current collector werepre-treated in the catholyte solution as described in Example #8.

Voltage-time discharge/charge profiles and delivered capacity vs. cyclenumber plots are shown in FIG. 15. Under described test conditions, thecell demonstrated several discharge-charge cycles.

Example 11

The cell was similar to the one described in Example #7. However, inthis case a carbon anode was used instead of a lithium metal anode, andthe aqueous catholyte contained 3M Li₂S (Catholyte #4). The anode was acommercial carbon electrode comprising synthetic graphite on a coppersubstrate and was similar to carbon electrodes commonly used inlithium-ion batteries. The non-aqueous electrolyte interlayer contained1M of LiTFSI salt dissolved in the mixture of ethylene carbonate anddimethyl carbonate (1:1 by volume). The assembled cell with thefollowing structure: carbon anode/non-aqueous electrolyte/glass-ceramicmembrane/aqueous Li₂S catholyte/carbon cathode exhibited an open circuitvoltage of −0.63V.

First, the cell was galvanostatically charged at a current density of0.1 mA/cm² for 20 hours. At the end of the charge, the cell voltagereached approximately 2.4V. Then, the cell was discharged at 0.1 mA/cm²to a voltage cut-off of 2.1V. The same charge/discharge procedure wasused for further cycling: the cell was charged at 0.1 mA/cm² for 20hours and then discharged at 0.1 mA/cm² to 2.1V.

FIG. 16 demonstrates that a cell employing a carbon anode and an aqueouselectrolyte containing Li₂S can work reversibly. This is the first knownexample of an aqueous solution containing lithium sulfides orpolysulfides being used as a source of Li cations for charging of acarbon anode. Therefore, aqueous catholytes containing active sulfurspecies can be used in combination with lithium intercalation compoundsin rechargeable lithium-sulfur batteries.

Example 12

The cell, pre-treated cathode and cycling procedure were the same asdescribed in Example #10. However, the catholyte contained 6M S havingan active stoichiometric ratio of Li₂S₄ in methanol (Catholyte #5,described above).

Voltage-time discharge/charge profiles and delivered capacity vs. cyclenumber plots for Li/S cells with a cobalt cathode and methanol-basedsulfur-containing catholyte are shown in FIG. 17. Under the describedtest conditions, the cell demonstrated several discharge-charge cycles.This is the first known example of a rechargeable Li/S cell with acatholyte based on a protic nonaqueous solvent.

Conclusion

Various embodiments of the invention have been described. However aperson of ordinary skill in the art will recognize that variousmodifications may be made to the described embodiments without departingfrom the scope of the claims. Accordingly, the present embodiments areto be considered as illustrative and not restrictive, and the inventionis not to be limited to the details given herein.

1. An aqueous lithium sulfur electrochemical cell comprising: an anodestructure comprising an electroactive material; a cathode comprising asolid electron transfer medium; an aqueous electrolyte in contact withthe electron transfer medium; and active sulfur species in contact withthe aqueous electrolyte; wherein the anode electroactive material isisolated from direct contact with the aqueous electrolyte.
 2. Theaqueous lithium sulfur cell of claim 1 wherein the active sulfur speciescomprises active sulfur species dissolved in the aqueous electrolyte. 3.The aqueous lithium sulfur cell of claim 2 wherein the active sulfurconcentration in the aqueous electrolyte prior to initially operatingthe cell is selected from the group consisting of a value that isgreater than 0.5 molar sulfur, 1 molar sulfur, 2 molar sulfur, 3 molarsulfur, 4 molar sulfur, 5 molar sulfur, 6 molar sulfur, 7 molar sulfur,8 molar sulfur, 9 molar sulfur, 10 molar sulfur, 11 molar sulfur, and 12molar sulfur.
 4. The aqueous lithium sulfur electrochemical cell ofclaim 2 wherein just prior to initially operating the cell: i) theconcentration of the dissolved active sulfur species in the electrolyteis greater than 1 molar sulfur; and ii) the active stoichiometric ratiois represented by the chemical formula selected from the groupconsisting of Li₂S; Li₂S_(x) (x>1); Li₂S_(x) (1<x<5); Li₂S₅; andLi₂S_(x) (x>5).
 5. The aqueous lithium sulfur electrochemical cell ofclaim 2 wherein just prior to initially operating the cell: i) theconcentration of the dissolved active sulfur species in the electrolyteis greater than 3 molar sulfur; and ii) the stoichiometric ratio ofactive sulfur to lithium dissolved in the electrolyte, exclusive of anynon-active lithium sulfur salts, is represented by the chemical formulaselected from the group consisting of Li₂S_(x) (2≦x≦5).
 6. The aqueouslithium sulfur electrochemical cell of claim 2 wherein just prior toinitially operating the cell: i) the concentration of the dissolvedactive sulfur species in the electrolyte is greater than 1 molar sulfur;and ii) the stoichiometric ratio of active sulfur to lithium dissolvedin the electrolyte, exclusive of any non-active lithium sulfur salts, isrepresented by the chemical formula Li₂S.
 7. The aqueous lithium sulfurelectrochemical cell of claim 2 wherein just prior to initial celloperation the active sulfur species in contact with the aqueouselectrolyte further comprises solid phase lithium sulfide.
 8. Theaqueous lithium sulfur electrochemical cell of claim 7 wherein the molesof active sulfur of solid phase lithium sulfide is greater than themoles of active sulfur dissolved in the electrolyte by a factor selectedfrom the group consisting of at least 2, at least 3, at least 5 and atleast
 10. 9. The aqueous lithium sulfur cell of claim 7 wherein the fullcharge capacity of the cell is derived from the dissolved active sulfurspecies and the solid phase lithium sulfide.
 10. The aqueous lithiumsulfur electrochemical cell of claim 1 wherein upon cell fabrication andprior to initial cell operation the anode electroactive material issubstantially devoid of active lithium. 11.-45. (canceled)
 46. A methodof making an aqueous lithium sulfur cell, the method comprising thesteps of: i) providing a lithium anode structure comprising anelectroactive material; ii) providing a cathode comprising an electrontransfer medium for electro-reducing active sulfur species during celldischarge; iii) providing an aqueous electrolyte comprising water; andiv) configuring the electrolyte, anode structure and cathode into abattery cell; wherein at least one or both of the cathode andelectrolyte comprises active sulfur species.
 47. The method of claim 46wherein the electrolyte comprises dissolved lithium sulfide.
 48. Themethod of claim 47 wherein the cell further comprises solid phaselithium sulfide in contact with the electrolyte.
 49. The method of claim46 wherein the cathode is devoid of elemental sulfur.
 50. The method ofclaim 46 further comprising the step of charging the cell, wherein saidcharging step is the initial electrochemical operation, and whereby thecharging reaction involves electro-reductive lithiation of the anodeelectroactive material and electro-oxidation of dissolved active sulfurspecies at the surface of the electron transfer medium.
 51. The methodof claim 50 wherein the anode electroactive material is devoid ofelectroactive lithium prior to the start of the initial charging step.52. The method of claim 51 wherein the amount of active sulfur speciesand lithium ions in the as-fabricated cell is sufficient for providingthe necessary ampere-hour capacity to fully charge the cell during theinitial charging step.
 53. (canceled)
 54. The method of claim 46 whereinthe anode electroactive material is an intercalation material capable ofintercalating lithium ions whence electro-reduced.
 55. (canceled) 56.The method of claim 46 wherein anode electroactive material is lithiummetal or a lithium metal alloy. 57.-65. (canceled)
 66. A lithium sulfurflow cell system comprising: a lithium sulfur cell comprising: an anodestructure comprising an electroactive material; a cathode comprising asolid electron transfer medium; an aqueous electrolyte in contact withthe electron transfer medium; active sulfur species in contact with theaqueous electrolyte; wherein the anode electroactive material isisolated from direct contact with the aqueous electrolyte; and furthercomprising: an external reservoir system comprising a storage tankcomprising aqueous catholyte in flow communication with the aqueouselectrolyte in the cell, the aqueous catholyte in the storage tankcomprising one or more of a dispersion of solid elemental sulfur,dissolved elemental sulfur, dissolved lithium polysulfide species,dissolved lithium sulfide, and a dispersion of solid lithium sulfide.